Vocabulary in English Language Learning, Teaching, and Testing in Vietnam: A Review

: This review paper aims to provide an overview of vocabulary in English language learning, teaching, and testing in Vietnam. First, we review studies on the vocabulary knowledge of Vietnamese EFL learners. Recent research evaluating different aspects of vocabulary knowledge shows that Vietnamese EFL learners generally have limited knowledge of both single words and formulaic language. Next, we discuss contemporary approaches to teaching vocabulary in Vietnam to reveal current issues and provide relevant recommendations. Empirical studies on Vietnamese EFL learners’ vocabulary acquisition are also discussed with an aim to shed light on how vocabulary can be acquired by Vietnamese EFL learners and subsequently draw important pedagogical implications. In addition, we look into the lexical component of high-stakes English tests in Vietnam, calling for more attention to the lexical proﬁles and lexical coverage of those tests. Finally, we provide concluding remarks and research-informed recommendations for EFL vocabulary learning and teaching in Vietnam to elaborate on how vocabulary can be effectively learned and taught.


Introduction
At present, English is undoubtedly the most popular foreign language in Vietnam due to the increasing international economic integration of the country. It is a compulsory subject in the curriculum of secondary education, i.e., from Grade 6 (12 years old), across Vietnam and is also introduced as a subject from Grade 3 (9 years old) in many provinces. In addition, its use as a medium of instruction at tertiary institutions, both public and private, has increased recently. The importance of English in Vietnam can also be seen in the growing number of English language centers throughout the country as well as the large investment of Vietnamese parents in expensive English courses. Nonetheless, the English language teaching (ELT) quality in Vietnam tends to be low [1]. This is partly reflected in the fact that a large number of Vietnamese learners of English as a foreign language (EFL) are unable to communicate in English for survival needs despite many years of formal English instruction, as suggested by a recent survey [2]. Therefore, the National Foreign Language Project 2020, a multimillion nationwide project, has recently been launched to bring about reforms in the learning and teaching of English and other foreign languages in Vietnam, aiming to enhance the foreign language competence of Vietnamese graduates of post-secondary institutions as well as the workforce [3].
Since the early 1990s, communicative language teaching has become popular in Vietnam because traditional pedagogy, which focused on grammar and vocabulary acquisition in isolation rather than communicative competence, i.e., "the ability to use language they are learning appropriately in a given social encounter" [4] (p. 3), did not satisfy the requirements of English learning in the new era [4]. In reality, however, the emphasis of ELT in Vietnam is still primarily on reading and grammar [5]. Vocabulary might receive some attention in the English curriculum in Vietnamese schools, but mostly to facilitate reading and grammar exercises. Considering the importance of vocabulary for second/foreign language (L2) learning and teaching [6,7], vocabulary arguably deserves a place of its own in the English curriculum in Vietnam. Additionally, not until recently has ELT research in Vietnam begun to pay attention to vocabulary.
To date, a number of topics related to vocabulary in ELT in Vietnam have been investigated. In particular, the vocabulary knowledge of Vietnamese EFL learners has attracted considerable attention [8][9][10][11]. In addition, several studies have examined Vietnamese EFL learners' vocabulary learning from extensive reading [12][13][14], audiovisual input [15], input-based and output-based tasks [16,17], computer-assisted language learning [18], and explicit instruction [19]. These studies, while still limited in quantity, have revealed intriguing findings that have significant implications for ELT stakeholders in Vietnam. This review paper provides an overview of vocabulary in English language learning, teaching, and testing in Vietnam. The paper begins with a review of studies that evaluate Vietnamese EFL learners' knowledge of vocabulary, followed by a discussion of contemporary approaches to teaching vocabulary in Vietnam. Next, we elaborate on empirical studies on Vietnamese EFL learners' vocabulary acquisition before zooming in on the lexical component of high-stakes English language tests in Vietnam. To conclude, we offer research-informed recommendations for vocabulary learning and teaching in Vietnam and similar EFL contexts.

Vocabulary Knowledge of Vietnamese EFL Learners
Recent research [8][9][10][11] has indicated that Vietnamese EFL learners generally have limited vocabulary knowledge, that is, understanding the meaning of English words and formulaic sequences (e.g., collocations, idioms) and knowing their form.
As for single words, Vu and Nguyen [10] conducted a large-scale study to examine the knowledge of meaning recognition of 500 Vietnamese EFL students in Grade 12 (18 years old) across different high schools in different provinces of Vietnam. The results of a vocabulary levels test revealed that only a very small proportion of Vietnamese twelfthgraders mastered the 2000-word level (14%) and even fewer students mastered the next word levels, even though their number of years of studying English ranged from 6 to 12 years. The findings further showed that students in rural areas had significantly poorer vocabulary knowledge than those in urban areas, which might have resulted from the differences in their exposure to English, parental guidance, goal setting, motivation, and available resources. Additionally, the vocabulary knowledge of male students was more limited than that of female students. Similar to Vu and Nguyen [10], Dang [11] also found that over 90% of Vietnamese non-English majored university students in a General English program did not master the most frequent 2000 words after 10 years of formal English instruction.
Nguyen and Webb [8] investigated Vietnamese EFL learners' knowledge of both single words and collocations (e.g., make a decision). A form recognition test on collocations and a vocabulary level test on meaning recognition of single words were administered to 100 university English majors whose English proficiency levels were between preintermediate and upper-intermediate. The results showed that the participants showed mastery of the 1000-word level but not of the next word levels, despite receiving formal instruction for at least 7 years. The authors suggest that there is a need for institutional vocabulary learning programs to expand learners' knowledge of high-frequency words. In addition, the study indicated that Vietnamese EFL learners possessed limited collocation knowledge. The probable reasons for the learners' poor knowledge of collocations were, as the authors explained, a lack of focus on teaching multi-word units, a lack of awareness of the importance of collocations, and an underestimation of the difficulty of collocation acquisition.
Another study focusing on formulaic language is the study by Tran [9], who examined the knowledge of form recall, meaning recognition, and meaning recall of 50 highly frequent, figurative idioms (e.g., a piece of cake, on top of the world) of 70 Vietnamese students with pre-intermediate and intermediate levels of English proficiency in one Vietnamese university. The results indicated that most of those common idioms were unknown by the students even though they were aware of the importance of idioms and idiom learning in EFL learning. Focus groups with the participants further revealed that they rarely used idioms because of a lack of attention to idioms in their English classes, a heavy focus on single words over multi-unit words, and insufficient repetition and practice.
The results of previous studies on Vietnamese learners' vocabulary knowledge are quite concerning. Research has consistently shown a strong, positive relationship between vocabulary knowledge and all four language skills of reading, writing, listening, and speaking [20]. As Wilkins [21] put it: "Without grammar very little can be conveyed; without vocabulary nothing can be conveyed" (pp. 110-111). Vietnamese learners' limited vocabulary knowledge might be one of the reasons why so many of them struggle with communicating in English after many years of instruction. This is also illustrated in Vietnamese students' alarmingly low scores in recent national high-school English examinations. For example, in 2019, almost 70% of Vietnamese students scored below average in the national high-school English exam [22]. All these issues might have resulted-at least to some extent-from learners' poor vocabulary knowledge.
There are a number of factors that might explain the limited vocabulary knowledge of Vietnamese EFL learners as L2 vocabulary learning can be affected by word-related factors, contextual factors, and learner-related factors [23]. Word-related factors include cognates, words that are similar in form or meaning, word length, parts of speech, concreteness and imageability, polysemy, and homonymy. Contextual factors consist of frequency of occurrence, first language (L1) frequency, and L2 frequency, while learner-related factors involve learners' prior vocabulary knowledge and working memory (see Peters [23] for a more detailed overview). All those factors might, to some extent, exert an influence on Vietnamese EFL learners' vocabulary learning and knowledge.

Vocabulary Teaching in Vietnam
Even though vocabulary acquisition processes are complex and explicit vocabulary instruction in class can only cover a small proportion of words students learn [24], it is still important to understand contemporary vocabulary teaching practices in Vietnam before any implications and recommendations are provided. Unfortunately, no studies to date have explicitly and systematically investigated English vocabulary teaching practices in Vietnam. What can be found in the literature is some mentions of vocabulary teaching techniques reported by different Vietnamese EFL teachers, such as translation [25], code-switching [26], songs [27], or storytelling [28]. The detailed accounts of how these techniques are implemented in real English classrooms and their effectiveness for vocabulary teaching, however, have not been documented. The teachers in those studies only reported that they used those techniques in their English classes. Little is known about the rationale behind Vietnamese EFL teachers' adoption of those vocabulary teaching techniques. Probably, Vietnamese EFL teachers either adopt those teaching techniques from their teacher training program or follow their model teachers [26].
A common procedure of teaching vocabulary can be found in many EFL classrooms in Vietnam. At the beginning of an English lesson, a Vietnamese EFL teacher may write on the board a list of new English words that will appear in the lesson, providing Vietnamese translations of those words, modeling the pronunciation of those words, and asking the whole class to repeat the words after the teacher several times. The focus is normally on single words, which are often selected either by intuition or based on glossaries in textbooks and reference books. After that vocabulary introduction, the focus often moves to a language skill, which is normally reading and grammar learning. In the following class, the teacher might call on some students to check their memorization of the words introduced in the previous class. However, learners are often left to their own devices when it comes to their vocabulary learning. They are encouraged to learn new words by heart but do not have much chance to put those words to use during the lessons because more emphasis is usually placed on reading and grammar. A note is that Vietnamese EFL teachers seldom assess their students' vocabulary knowledge before and after an English course to measure their students' vocabulary gains. Instead, they often tend to prioritize grammar knowledge.
Such approaches to teaching vocabulary are problematic in several ways (see Table 1 for a summary of the current approaches to teaching vocabulary in Vietnam as well as corresponding recommendations). First, many Vietnamese EFL teachers tend to focus heavily on the connection between the form and the meaning of words. Nation [6] suggests that knowing a word involves a number of aspects of vocabulary knowledge other than those three aspects, such as associations, collocations, and constraints on use. In addition, textbooks, which are one of the main sources of input for L2 learners, might inadequately address different aspects of vocabulary knowledge [29,30]. Thus, in addition to focusing on establishing form-meaning connections, Vietnamese EFL teachers should also address other aspects of vocabulary knowledge. Table 1. Current approaches to teaching EFL vocabulary in Vietnam and relevant recommendations.

Recommendations
Teachers usually focus primarily on form-meaning connection.
Teachers should pay more attention to different aspects of vocabulary knowledge (e.g., associations, collocations, constraints in use). Teachers usually select vocabulary based on their intuition or glossaries in textbooks and reference books.
Teachers are advised to consider the frequency of vocabulary when selecting vocabulary.
Teachers usually pay attention to single words. Teachers should also devote adequate attention to formulaic sequences (i.e., collocations, idioms). Teachers usually do not evaluate learners' vocabulary knowledge before and after a course.
Teachers can assess learners' vocabulary knowledge (e.g., with a bilingual vocabulary test) before and after a course. Teachers usually teach vocabulary deliberately without providing learners with opportunities to encounter and use it in different contexts.
Teachers should give learners opportunities to encounter and use vocabulary across four strands: meaning-focused input (reading, listening, viewing), meaning-focused output (speaking, writing), language-focused learning (direct teaching, vocabulary learning strategy training), and fluency development (easy activities to enhance learners' access to existing knowledge). Teachers usually do not train learners in effective vocabulary learning strategies.
Teachers should encourage learners to encounter and use English outside the classroom. Teachers can also train students to learn word parts, guess meanings from context, and use dictionaries and flashcards effectively.
Second, the frequency of a word is an important criterion for vocabulary selection [7]; thus, it is advisable that Vietnamese EFL teachers take into consideration the frequency of lexical items they intend to introduce to their students rather than merely base their vocabulary selection on their intuition or glossaries in textbooks and reference books. They can familiarize themselves with such programs as Lextutor [31] or AntwordProfiler [32] to analyze target words to make sure that those target words deserve teaching time in the classroom.
Third, formulaic sequences are ubiquitous in discourse [33] and crucial for L2 skills [34,35]. Therefore, formulaic sequences deserve more teaching time, and Vietnamese EFL learners would benefit from a stronger focus on formulaic sequences.
Fourth, Vietnamese EFL teachers should assess their students' vocabulary knowledge at the beginning of each course to design it suitably and at the end of the course to evaluate their students' vocabulary learning. They can use a bilingual vocabulary size test [36] for this purpose.
In addition, Vietnamese EFL teachers' deliberate teaching of vocabulary can help students establish the form-meaning relationship of target words, but such a way of teaching may not provide sufficient opportunities to encounter and use vocabulary in different contexts. Nation's Four Strands [37] provide a valuable framework for guiding teachers. These four strands are meaning-focused input, meaning-focused output, language-focused learning, and fluency development. Simply put, apart from deliberate vocabulary teaching, which addresses language-focused learning, Vietnamese EFL teachers can help enhance their students' vocabulary knowledge from meaning-focused input (i.e., reading, listening, and viewing) as well as meaning-focused output (i.e., speaking and writing). Vietnamese EFL teachers should also provide learners with easy and familiar activities that aim to enable learners' quick access to existing knowledge in order to develop their fluency in each of the four skills of listening, speaking, reading, and writing (see [7] for more elaborations on how to use these four strands to develop an effective vocabulary learning program in a language course).
An important feature of the language-focused learning strand, which is also an issue that Vietnamese EFL teachers generally seem not to pay attention to, is vocabulary learning strategy training. Teaching students effective vocabulary learning strategies can help them become autonomous learners [7]. The following vocabulary learning strategies, which are recommended by Webb and Nation [7], can be useful for Vietnamese EFL teachers to train their students.
First, Vietnamese EFL teachers can encourage their students to find ways to encounter English outside the classroom. Since students have a limited number of hours to learn English inside the classroom every week, it is important that they are also exposed to English outside the classroom. Indeed, exposure to L2 input is crucial for vocabulary learning [38,39]. Extensive reading (i.e., reading in large amounts for pleasure) and extensive viewing (i.e., watching television or online videos) have been proven to be useful resources for vocabulary learning [12][13][14][40][41][42]. Vietnamese EFL teachers can educate their students about the importance of language input for language learning and also train them to adopt it effectively.
Second, Vietnamese EFL teachers are recommended to encourage their students to attempt to use English outside the classroom. Classroom activities can help students develop their receptive knowledge of vocabulary but may not help them use words fluently [7]. For that reason, Vietnamese EFL teachers can stress the importance of using vocabulary regularly in speaking and writing and suggest activities that can help their students practice using vocabulary outside the classroom. For example, Webb and Nation [7] suggest writing blogs, texts, and emails for writing activities and discussion groups on social media or spoken journals for speaking activities to increase students' opportunities for vocabulary use.
Other vocabulary learning strategies that Vietnamese EFL teachers can train their students include learning word parts (i.e., prefixes and suffixes), guessing word meaning from context, using dictionaries effectively, and using flashcards (see [7] for more information). Considering the large number of words for students to learn, it is vital that Vietnamese EFL students can become autonomous in their own vocabulary learning. Vietnamese EFL teachers can play an important role in that process by training their students to use the above-mentioned vocabulary learning strategies.

Research into Vietnamese EFL Learners' Vocabulary Acquisition
Because of the significance of vocabulary in L2 learning, a number of studies have investigated how vocabulary can be acquired by Vietnamese EFL learners. While research on L2 vocabulary learning in other contexts might be useful and applicable, studies that have a specific focus on Vietnamese EFL learners have their own values, because language learning might vary among learners of different contexts. To date, such studies have focused on the effects of different types of input [12][13][14][15], focused tasks [16,17], computerassisted language learning [18], and explicit instruction [19] on Vietnamese EFL learners' vocabulary acquisition.
First, considering the importance of exposure to input for vocabulary learning [38], studies on the effects of different types of input on Vietnamese EFL learners' vocabulary acquisition are necessitated. Nguyen and Boers [15] compared the effects of content retelling on vocabulary learning from a TED talk among two groups. One experimental group of Vietnamese EFL students watched a TED talk video, orally summed up its content in English, and then watched the video again, while a comparison group only watched the video twice without summing up its content. The findings revealed that both groups showed gains in meaning recall tests, but the gains in the experimental group were greater than those in the comparison group. Words were even better recalled later if they were used in the summaries. This study highlights the potential of using audiovisual input (i.e., TED talks) for vocabulary learning.
Addressing another kind of input, Vu and Peters [12,13] looked into the effects of extensive reading in three conditions: reading-only, reading-while-listening, and reading with textual input enhancement (i.e., underlining) on learners' uptake of single words and collocations. The results showed that extensive reading in any of those reading conditions resulted in significant gains of single words and collocations among learners. Reading with textual input enhancement led to significantly more vocabulary gains than the other reading conditions. In another follow-up study, Vu and Peters [14] compared the effects of readingwhile-listening, reading with textual input enhancement, and reading-while-listening plus textual input enhancement on incidental collocation learning. The findings revealed that both reading with textual input enhancement and reading-while-listening plus textual input enhancement led to superior gains compared with reading-while-listening. Studies by Vu and Peters [12][13][14] and Nguyen and Boers [15] clearly demonstrate the potential of extensive reading and extensive viewing for Vietnamese EFL learners' vocabulary learning.
However, Laufer [43] argues that merely comprehensible input is not sufficient for vocabulary learning. Indeed, Hoang and Boers [44] showed that learners only attempted to recycle a small proportion of multiword expressions from an input text, and a considerable number of those expressions were incorrect. Laufer [43], therefore, suggests that focus on form, i.e., paying attention to linguistic aspects, should be a crucial part of instruction. She recommends that focused tasks, which are designed to "elicit the use of specific linguistic forms in the context of meaning-centered language use" [45] (p. 420), can be used for vocabulary teaching. Duong et al. [16,17] conducted empirical studies on the effects of two types of focused tasks, i.e., input-based and output-based tasks, on Vietnamese EFL learners' vocabulary uptake. In Duong et al. [16], for input-based tasks, learners read L1 emails enquiring about travel tips, watched L2 captioned videos, and then wrote emails in Vietnamese answering the questions from L1 emails using information from L2 videos to show their understanding of the target items. In output-based tasks, learners wrote an L2 travel blog and wrote an L2 email proposing a travel itinerary with an English-Vietnamese picture glossary provided. The findings showed that both task types resulted in vocabulary uptake, but input-based tasks resulted in higher gains for learning the meaning of English vocabulary, and output-based tasks led to more gains for learning the form of English vocabulary. The positive effects of both task types on vocabulary learning were also reported in Duong et al. [17], which focused on spoken tasks and spoken vocabulary knowledge.
Technological developments have given rise to a number of new ways for vocabulary learning and teaching. Data-driven learning, in which learners learn a language through interactions with data from a corpus using different tools, is one of such innovative ways. Bui, Boers, and Coxhead [18] investigated a classroom intervention where Vietnamese EFL learners in an experimental group looked for multiword expressions in texts in an online corpus and consulted online dictionaries. Compared with a control group that only used the same texts for activities on their content, the experimental group showed significantly larger gains of multiword expressions in the post-test one week later. The intervention was particularly effective for learners with high English proficiency. This study showed that data-driven learning can be an efficient way for Vietnamese EFL students to improve their vocabulary.
Another study looked into the effect of instruction on vocabulary learning. Le, Rodgers, and Pellicer-Sánchez [19] examined the effectiveness of different teaching approaches on Vietnamese low-proficiency EFL learners' learning of form and meaning of formulaic sequences. Learners in two experimental groups were explicitly taught formulaic sequences either with or without sentence context while learners in one control group were only exposed to formulaic sequences in coursebook instruction without explicit teaching. The findings demonstrated that the two experimental groups learned formulaic sequences significantly better than the control group, indicating that explicit instruction is more effective for the learning of formulaic sequences than coursebook instruction. Sentence context and salience of formulaic sequences, however, had no significant effects on the learners' acquisition of those formulaic sequences.
In general, the increasing number of studies on Vietnamese EFL learners' vocabulary acquisition can make important contributions to vocabulary learning and teaching in Vietnam. Those studies have significant implications for ELT in Vietnam, providing useful suggestions of how vocabulary can be learned and taught inside and outside the classroom.

Vocabulary in English Language Testing in Vietnam
Research has indicated the relationship between vocabulary and comprehension [46]. Lexical coverage, i.e., the proportion of words known in a text, is an important factor that can affect comprehension. Laufer and Ravenhorst-Kalovski [46] suggest 95% coverage for minimal comprehension and 98% coverage for optimal comprehension. In other words, learners need to know at least 95% of the words in a text to gain a minimal understanding of that text. This threshold is clearly important for not only reading but also language tests. According to Webb and Paribakht [47] (p. 41), research on lexical profiling can "provide useful vocabulary learning goals that language learners can aim for" while studies of lexical coverage show "the importance of vocabulary knowledge for reading and listening comprehension". To date, however, very few studies have investigated the lexical coverage of language tests.
Among the few studies on English language tests in Vietnam and the first study to analyze their lexical content, Vu [48] conducted a corpus-based lexical analysis of 20 English exam papers for university admission and high-school graduation in Vietnam during a period of 17 years (2002-2018). The results showed that the exam papers required a vocabulary knowledge of 5000 word families to reach 95% coverage. Different exam papers required different amounts of vocabulary knowledge, ranging from 3000 word families up to 12,000 word families for 95% coverage, which indicates substantial variations among the exams. The study questioned whether such high lexical demands of those high-stakes exams were suitable for Vietnamese high-school students given their limited vocabulary knowledge [10].
Vu's study [48], however, does not suggest that high-stakes English tests in Vietnam should specifically focus on vocabulary only. Rather, when designing high-stakes English tests in Vietnam, test designers should pay more attention to the lexical profiles and lexical coverage of the tests so that those tests can be more consistent, aligned with the policies of the MOET [49], and suitable for Vietnamese EFL learners [10]. A language test that requires a too-large vocabulary might be too difficult for many learners, and it can be arguably demotivating to them as well.

Conclusions and Recommendations
Apparently, vocabulary research in Vietnam has been far and few between; therefore, the picture of vocabulary in English language learning, teaching, and testing in Vietnam is far from complete at the moment. What seems clear at present is that a more principled approach to vocabulary is required for English language learning, teaching, and testing in Vietnam.
To improve vocabulary learning and teaching, both Vietnamese EFL learners and teachers should be aware of the conditions required for vocabulary learning. These conditions include repetition and quality of attention [7]. More specifically, as regards repetition, learners should encounter a word many times to acquire it. It is important to note that different words might require different numbers of repetitions, but research has recommended eight, ten, or even more times for incidental vocabulary learning to take place [50][51][52]. Quality of attention also has an important role to play [53]. According to Webb and Nation [7], four conditions contributing to the quality of attention include noticing, retrieval, varied encounters and varied use, and elaboration. Learners need to pay attention to a word (i.e., noticing) first and then retrieve its meaning while reading or listening, or its form while speaking or writing (= opportunities for retrieval). Next, learners need to encounter and use a word in different forms or contexts (i.e., varied encounters and varied use). Finally, they need to be exposed to different aspects of a word, including its form, meaning, and use (i.e., elaboration). If those learning conditions for vocabulary learning can be met, they can potentially optimize learners' vocabulary learning process.
In addition, as discussed in Section 3 above, an important consideration in vocabulary learning and teaching is the selection of vocabulary, as not all words are created equal. It is high-frequency words that deserve the most learning and teaching time because of their value for communication. Learners are more likely to encounter and use highfrequency words compared to low-frequency words because high-frequency words occupy a large proportion in both spoken and written discourse [7]. For example, Nation [54] analyzed the British National Corpus and found that a knowledge of between 3000 and 4000 most-frequent word families can provide up to 95% coverage, i.e., the percentage of words necessary for comprehension, of different kinds of both spoken and written texts while knowledge of between 6000 and 9000 most-frequent word families can provide 98% coverage. In other words, knowledge of high-frequency words can help learners improve their comprehension substantially. Meanwhile, low-frequency vocabulary, i.e., words beyond the most frequent 9000 words (e.g., proper names, technical vocabulary), only provides a low coverage of any text [6]. Therefore, learning high-frequency words is a useful learning goal for L2 learners (see [6] for a more thorough discussion on the goals of vocabulary learning).
To facilitate the learning and teaching of high-frequency words, frequency-based word lists can be a valuable resource. One of the useful frequency-based vocabulary lists is Nation's BNC/COCA2000 high-frequency word list [55], which has been found to be suitable for EFL learners in Vietnam and probably for many other EFL contexts as well [56]. Other relevant word lists are the General Service List [57] and the New General Service List [58] or the Academic Words List (specialized vocabulary for academic purposes) [59,60]. Vu [48] showed that the General Service List [55] accounts for 85.87% of Vietnam's university admission and high school graduation English exams (from 2002 to 2018), which are two high-stakes exams that a large number of Vietnamese students have to take, while the Academic Word List [59] covers 5.06% of the exams. In addition to the lists of single words, there are also lists of highly frequent formulaic sequences [60,61]. In order for high-frequency words to be effectively taught and learned, learners need to encounter those words across the four strands of meaning-focused input, meaning-focused output, language-focused learning, and fluency development (see Section 3). It should be noted, though, that merely high-frequency vocabulary is not sufficient for learners to read unsimplified text without assistance. Attention is also required for mid-frequency vocabulary, i.e., 7000 word families from the third 1000 words to the ninth 1000 words, which could be addressed with learner strategies such as guessing from context, using vocabulary cards, or dictionaries [6]. Meanwhile, low-frequency vocabulary can probably be best learned incidentally from reading or listening [6].
In addition, as suggested by empirical studies on Vietnamese EFL learners' vocabulary acquisition elaborated in Section 4 above, there is great value in learning vocabulary from extensive reading or extensive viewing [12][13][14][15]. Therefore, Vietnamese EFL teachers can either adopt those activities in their language classrooms or recommend those activities to their students as informal learning activities outside the classroom. As for reading, textual input enhancement (e.g., underlining) can help learners pay more attention to target lexical items and learn them from reading better [12][13][14], and reading-while-listening can also be a possible option at learners' disposal. Data-driven learning can also be adopted if technology is readily available to learners [18]. When it comes to instruction, explicit vocabulary instruction can also be more effective than mere coursebook instruction [19]. Moreover, to enhance learners' vocabulary acquisition from comprehensible input, teachers can employ both input-based and output-based tasks, which can help bolster different aspects of vocabulary knowledge [16,17].
Given the importance of vocabulary for reading, listening, speaking, and writing, as well as an increased interest in research on vocabulary learning and teaching [6], it is hoped that more studies on vocabulary learning and teaching in Vietnam will be conducted to inform interested stakeholders and relevant ELT policies of the country.