Originally Posted by
Big D
I don't mind Vaan.
To me, his main role seems to be as a storytelling device: he's young and rather naive, so he doesn't know a whole lot about Ivalice. By explaining its history and geography to Vaan, the other characters are actually explaining it to us. Much like in The Lord of the Rings, when the heroes arive at a new town, ruin or grassy knoll and Legolas decides to regale them with over 9000 poems detailing that location's origins and culture.
Vaan's other big purpose is to be a playable incarnation of an 'average person' caught up in Dalmasca's conflict. With the exception of Penelo, the other main characters are all rather extraordinary in their own right: Ashe is the fallen princess, Basch the condemned 'traitor', Baltheir the son of Archadia's mad genius and a former Judge. Vaan, though, is a lot more down-to-earth: a young Dalmascan, not a solider but still a victim of war, losing his only brother and his nation's freedom. As he starts to expand his horizons and realise his dreams over the course of the game, Dalmasca does too. In that way, Vaan allows us to see first-hand a nation's growth as well as a man's.
So, it's fair to say that he isn't too crucial to the plot - but he contributes to the plot and the storytelling simply by being caught up and carried along in events. He has a legitimate motivation for joining the fight, and undergoes genuine growth in the process.