"너는 왜 채식주의자야?"
Translation:Why are you a vegetarian?
I quite agree. The word "vegetarian" can be used as noun and adjective. Though "Why are you a vegetarian?" (noun) is one of correct answers, "Why are you vegetarian?" (adjective) would be preferred in general and should be accepted. Reported on Nov. 12, 2017.
i think it's along the lines of what you said, that "a vegetarian" clearly means it's a noun, and just saying "you vegetarian" would mean it's an adjective.
I think because that way would make it an adjective, that it would be said differently.