i think it was made that way on purpose to show a point that its not always about one person. but then again they need someone to be the one in towns and since vaan is the first you meet he gets the role of the one who walks around towns.
as for whether or not he was the main character id say balthier was more the main character then vaan. he lead the group everywhere and after all he was the "leading man". he knew more about everything then everyone else. also in the end he was more the hero then anyone else.
and vaan did have a heroic scene or 2. when he caught ashe off the ledge. and if i remember correctly he did pick up gabranths sword and ram him.