Well ... I have not been teaching English (teach learners how to speak/use the language itself, not Linguistics or ELT Methodology, or EL Testing in the language), for a long time now, so I cannot answer the question without trying to refresh my memory first. And in this age of the Internet, how do you refresh your memory? Well, surfing the Net, of course.
And ... lo and behold! I found several good articles about teaching English pronunciation to Vietnamese students which are directly relevant to the question asked by my student.
For example, let's read this article from the Foreign Policy Journal (my first time at the site). A very good article, many good points, the only problem with it is that it is way too long, which may make many people give up right from the start. So, if you don't want to read all of it, below is an excerpt from the article that I find worthy of attention.
Listening: I am a proponent of ALG Automatic language Growth, a listening-only method of language acquisition. (You can watch some of my ALG Videos here).
Without going 100% into ALG or applying it exclusively to an ESL classroom in Vietnam, I believe, beyond any doubt, that a significant factor contributing to Vietnamese students having pronunciation issues is that they simply don’t listen enough. If you haven’t heard the sounds, you can’t reproduce them. In the commercial ESL marketplace, across Asia, parents are told that their children will be speaking English from their first day. The focus of the entire program is on speaking, rather than listening. Good foreign ESL teachers do model the target language, before asking students to produce it. But it’s not enough. When you learned English, you heard phrases hundreds or even thousands of times before you spoke them. But in Asian ESL, students are asked to produce after one or two hearings.
[How very true!]
Another article that I found very useful is this one, from a colleague in Ha Noi, here. An excerpt:
Below is a list of core items to teach (the first 7 are believed to be most essential):
1. The English alphabet. A focus should put be on the following letters which students confuse the sounds of: R, I, E, G , J, H, , K, Q, W, X, Y
2. Familiarisation with the English phonemic chart. Essential as it helps students to be able to know the pronunciation of words from dictionaries. Teachers should encourage students to use monolingual dictionaries made by reputable publishers.
3. Voiced and unvoiced sounds. Students should be taught this to help with the pronunciation of ‘s’ and ‘ed’ endings.
4. Long and short vowels. Students need to be able to confidently differentiate and produce these as they are both challenging and have an effect on meaning.
5. Word final consonants. Vietnamese students often neglect these and constant exercises on final endings should be done attentively during any course.
6. Consonant clusters. These are not a feature of Vietnamese and therefore are challenging. ‘sts’, ‘ts’, ‘str’, and ‘tr’ appear to be the most challenging for many students.
7. Suprasegmental level: Word stress, sentence stress, and intonation are essential items to address. Tonic intonation should be given special care as changes alters meaning. Sound linking is important but not essential as when learners say the words correctly, they will link sounds naturally themselves.
8. English sounds not found in Vietnamese. For example, the interdentals /d/ (voiced) and /th/ (unvoiced). /th/ can be mixed up with /f/ or Vietnamese "th" though this may not influence comprehensibility.
9. /l/ and /n/ can be mixed up in the northern dialect (Hai Phong, Hai Duong, Hung Yen, Quang Ninh etc.)
10. Initial /j/ like in yes, young, yellow may be heard as in zes, zoung, zeallow. This sound can be a bigger problem for learners from the south, or central provinces of the country.
11. /r/ The Hanoi accent does not distinguish between /r/, /z/, or /dj/. Some people in the central part of Vietnam such as Nghe An, Quang Binh, Hue, or Danang can say /zed/ instead of /red/.
12. The difference between aspirated and non-aspirated ‘t’. Initial ‘t’ in English is aspirated as in ten and tea. After ‘s’ as in stop and steel ‘t’ is not aspirated and is more similar to its Vietnamese counterpart. This is advisable to teach, but not on a short course.
Sounds not causing comprehension problems for the Vietnamese.
The sounds b, c, d, ch, f, k, h, m, n, ng, q, s, t, v, w may be a little difficult to produce but they don’t tend to lead to problems of comprehensibility.
Diphthongs do not seem to cause much difficulty for the Vietnamese.
Principles of teaching English pronunciation to the Vietnamese.
Pronunciation teaching should be systematic, gradual, consistent, interesting, practical, and integrated.
Well, there are a lot more out there, but I should not spoil it for you. You must go and check for yourself, really.
My final note: the Internet is such a treasure for anybody wanting to learn new things, and for learning English especially. Surf the Internet everyday, looking for things to read, and keep a blog in English (like this one), and your English will improve in no time at all!