I don't like him one bit, but I would definitely say Vaan is the 'main character' - but that's only because this is a video game, and for me the main character of a video game is the one that most things are seen from the perspective of, especially at the start.

But, as many have said, he's not a story character. To summarise: Main character of game is Vaan, main character of story is Ashe. The question in the thread title states game, so I voted Vaan.