"Вас" is in Akkusativ, it is used when some action in the sentence is done to the word (object) and not by the word (subject)
In this case the subject of the sentence that is doing the action is "ви", so it should be in Nominative. It is a bit subtle because you can't see the verb of the sentence, it's the hidden "to be". In English, "Where are you from?" --> "You are" --> "you" is the subject.