I've been teaching English to Vietnamese people for over 30 years and and I find Vietnamese English to be both amusing and frustrating. Because Vietnamese is monosyllabic, the Vietnamese tend to drop syllables or just forget them altogether. My girlfriend has a unique name for a refrigerator that works so well I think it would make a good replacement for the real word. Refrijator. Certainly California got a whack job with Cali, and so many words just lose their last syllable altogether. I've given up on trying to get my girlfriend to pronounce words correctly. It's so much easier just to let her speak Vietnamese. She went to a restaurant the other day and sampled some alligator. Her description of that was a riot. It was a mix of crocodile and alligator and was entirely undecipherable. I could've understood cá sấu right away but she felt she had to speak English.
How many of you understand Vietnamese English so well that you just let it slide and don't spend the time to help them get it right?
Vietnamese children are so bright and attentive that it is a pure joy to teach them. And unlike Angie, my SO, the language centers of their brain are still malleable and they will accept correction.
Vietnamese English
A quick follow up...
My wife and I adopted a Viet orphan in 1992. She is a good little cook and learned her way around our kitchen quickly. One day she came to me and said "em ush ee o dau?"
I was at a total loss. em ush ee, em ush ee, em ush ee, ?????
We finally found the bottle she was looking for. It was MSG - monosodium glutimate.
bột ngọt
That was so cute I'll never forget it.
As for me, you should correct them if their pronunciation is quite inaudible and undecipherable. In my opinion, most Vietnamese people do not feel ashamed and won't hesitate to fix their English pronunciation to speak better if foreigners tell them to do so 
)
Also, I appreciate every accent and no accent is more important than the others, it is not necessary to speak in British accent or American accent to be understood by people. But it's always better if one can pronounce clearer no matter what is his/her accent. So I think you'd better not let it slide and help them correct it right away 
In my experience, once some Vietnamese nationals are comfortable with their understanding of English it doesn't matter how much correction they receive. My efforts are not a result of laziness on my part but rather it is a complacency of the recipient. In the case of my SO, it is much easier to switch to communicating in Vietnamese. I am much more amenable to improving my Vietnamese language skills than she is to improving her English. The difference in the time and energy expended in the conveyance of information is profound. Your advice on dealing with this situation in an educational environment is sound.
It is possible that I should continue to try to communicate in English. I read that solving puzzles can ward off Alzheimer's.
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