We also don't have Singular in this sentence. It is vietnamese people in General. This question about General things is discussed many Times in duolingo. The answer Was, as I See it, always the same: generality is in english expressed with Plural.
I see 'How do Vietnamese learn English' as grammatically incorrect. How does 'How do American/Australian/English speak Vietnamese' sound? I argue you need 'the Vietnamese' when referring to the people as it is an adjective in this case.
Sorry, Mate, you are mistaken. "How do Vietnamese learn English" is perfectly grammatical. "Vietnamese" serves both as an adjective and as a noun in English. Without an article it is understood to be plural.
umm, the plural marker, nhung, was not in the sentence, so it should be "How does the Vietnamese person learn English?" I had it the "correct" way first, and then I changed it, because the nhung wasn't there. Not fair!