Vietnamese vs Asian

Vietnamese and Asian both are nouns.

Word	Noun	Adjective	Verb	Adverb
Vietnamese	Yes	Yes	No	No
Asian	Yes	Yes	No	No

As nouns, Asian is a hypernym of Vietnamese; that is, Asian is a word with a broader meaning than Vietnamese:

Other hypernyms of Vietnamese include Asiatic.

Vietnamese (noun)	Asian (noun)
the Mon-Khmer language spoken in Vietnam	a native or inhabitant of Asia
a native or inhabitant of Vietnam

Vietnamese (adjective)	Asian (adjective)
of or relating to or characteristic of Vietnam or its people or its language	of or relating to or characteristic of Asia or the peoples of Asia or their languages or culture