Probably, more than any other nation, Vietnamese learners have a very difficult time studying English. Here I will point Vietnamese pronunciation because this is the biggest issue. Vietnamese learners have particular difficulties with some or all of the following sounds: /f/, /θ/, /ð/, /z/, /ʃ/, /ʒ/, /ʧ/, /ʤ/, /p/, /b/, /l/.
The closest Vietnamese equivalent to /f/ is bilabial. Pronouncing the word “fifteen” is a difficult task. They will say “fipteen”. This is a problem not only for students, but for Vietnamese teachers of English as well. Everybody pronounces “fipteen” so, fixing this problem seems to be an impossible task.
Vietnamese learners tend to pronounce final stop (/p/, /t/, /k/) unexploded in all contexts. Not only these sounds, but almost all final sounds are missing from their speech. Words like “like”, “line”, “light”, “lie” are pronounce “lie” /laɪ/.
Initial /t/ is unaspirated in Vietnamese, producing a sound which can be confused with English /d/. However, there is a strongly aspirated initial /th/, written "th" in Vietnamese, which learners may produce as the equivalent of English "th" (/θ/, /ð/). I have been criticized many times by the Vietnamese English teachers for my /t/ sound which they say it sounds “too heavy” and I should pronounce it “smoothly” (these are exactly their words).
/g/ sound is frequently pronounced laxly without full closure, which gives it a guttural sound. /g/ sound in “egg” is not pronounced by Vietnamese learners.
Final /s/ is not pronounced when it should be and it is pronounced when it shouldn’t. Pronouncing plural form for a noun seems to be an impossible task.
Using “to be” is always a big problem. “My name Lam”, this is the way Vietnamese speak. Although they know the rule, they don’t apply it.
Vietnamese language is a mono syllable language (actually it isn’t but ...) which rises to mistakes. Producing multi syllable words is another difficult task. Simple words like “because” and “of course” are pronounced /bicɔ/ and /ɔfcɔ/, the second half of the word is not pronounced.
The word “like” is often pronounced “play” by the beginner learners. I couldn’t figure out why it is pronounced like that.
The –ed used for past tense at the end of the regular verbs is not pronounced. Vietnamese language doesn’t have such a suffix.
Two consonant sounds next to each other (not in initial position) also create big problems in pronunciation. Usually is chosen one of them and the other one is omitted.
Such mistakes persist and are very difficult to eliminate.
English seems to be quite simple, learners have a lot of opportunities to see good examples of English language (Internet, English TV channels etc) and English grammar is relatively simple. All these advantages are useless if we talk about Vietnamese learners.
At the first view, the main reason seems to be Vietnamese language itself. The highly complex Vietnamese vowel system possesses eleven pure vowels and many more diphthongs and triphthongs. Pure vowels are relatively few in English, but Vietnamese will frequently have recourse to the nearest Vietnamese pure vowel in pronouncing what they wrongly perceive to be a pure vowel in English: "lo" for low or "me" for may. Certain pairs of Vietnamese diphtongs are distinguished only by the length of the constituent vowels: "may" /mai/ meaning "to sew" and "mai" /ma-i/ meaning "tomorrow"; "nau" /nau/, "to take refuge" and "nao" /na-u/, "to make a disturbance".
Vietnamese language is a tonal language that means it uses tones. There are 5 tones in Vietnamese language which change the meaning of the word. It is very difficult to distinguish these tones for a non-native Vietnamese. They are applied to vowels only and their use is very important. The vowel in cause is pronounced as a long vowel, short vowel, with the voice raised up or down etc. Vietnamese speaker’s ears are trained to look for these sounds and they try to find these sounds in English language as well. English doesn’t have these kinds of tones and pronouncing them in English language doesn’t really matter and does not affect the meaning.
This frequently causes difficulty to Vietnamese learners, who hear a difference which is meaningful in Vietnamese and do not associate a word spoken by one English teacher with the same word spoken by another teacher having a different accent.
In my opinion this is the biggest problem for Vietnamese learners, a problem which is extremely difficult to fix.