The thing is, even if Vaan doesn't fit into the story in some grandiose fashion, I'd rather have a more lax protagonist this time around. I can't count the number of RPGs I've played wherein the seemingly unassuming and regular hero turns out to house some ancient, mysterious power that makes him the only weapon against the game's villainous antagonist. It's actually refreshing to me to see a hero that's just another dude along for the ride. That's kinda what Square was going for with Tidus, and mostly succeeded at. I like the idea of Vaan not necessarily being the "hero", but rather the avatar for the player. In essence, an inexperienced and mostly unknowledgeable kid that's sharing the learning experience right alongside you. Crono did the same thing, only he didn't talk.