Interesting, the word chay is from Chinese 齋 (Mandarin: zhāi, Cantonese jāai, Middle Chinese ʈʂɛj, Old Chinese tsˤr[ə]j).
I wonder if the Thai word for 'vegetarian food', เจ [ʨeː] is also derived from this same word...
The translation suggestions say that "người ăn chay" can mean both "a vegetarian" and "a vegan;" is there some convenient way of telling the difference in Vietnamese?
My guess is that there isn't other than adding another statement. Telling a vietnamese you're vegetarian is something which makes sense to them as its part of the buddhist tradition. Trying to explain veganism and they look at you like you have a third eye. There are some restaurants in HCMC which do claim to be vegan but I haven't noticed if they call themselves anything different from the normal vegetarians
It seems so since another lesson gave it that way. But let us not hold our breaths for a clarification on how to differentiate the use.