He's an everyman for the player to experience the story through. Tidus, despite being a sports celebrity, was one as well, but Vaan was a better execution of the idea. Ironically, that's what makes people find him so boring, which I honestly can't say I blame them for.

Personally I still agree with whomever on the team wanted it to be Basch or Balthier, but an even better choice would've been Ashe as it's primarily her story. Nevertheless, every time the characters explain stuff to Vaan and Penelo, they're explaining it to us, and that's extremely important, because without that, you're left with datalogs. And screw those.

Vaan gets points for not being particularly whiny or annoying; he's just a nobody who wants to find his place in the world, and in a very clever way, the game manages to give us that frustration by having us take him through countless quests and missions only for the established important people to get the attention.