"Vietnamese like to drink tea" is NOT ambiguous. It is plural and the plural verb clearly indicates plurality. "A Vietnamese likes to drink tea" is singular.
If that sounds natural to you it should be accepted, but it sounds quite weird to me without the word people.
It is different here, English people drink tea bag but Vietnamese people usualy drink green tea and dry tea.
Does this have to be plural? Can it not be " the Vietnamese person likes to drink tea"?
Yes but generally when you say người + nationality, it's talking about that nationality in general. To specify a particular Vietnamese person you'd say "người Việt Nam đó/kia"
"Vietnamese like ..." can only mean "Vietnamese people like... " It should be accepted. ("Like" in this sentence is plural)
In some ways. I personally find the lack of cases and conjugation refreshing.
First time I encountered this I answered "Vietnamese like..." and it was accepted. Not now. What changed in a day?