I am one of the few that actually liked at least the idea of Vaan's character: here's a kid who had let his circumstances act as an excuse for not doing anything productive with his life. He gets caught up in the plot to restore Dalmasca and in so doing is given the opportunity to develop talents that he would not have had he remained a street rat in Rabanastre. It is a coming of age story of sorts, though far more subtle than the heroic characters we are used to seeing.

That said, he is arguably worse-developed than most of the party (except perhaps Fran). And yes, he does become an irrelevant character from the standpoint of the game's primary plot.

His dialogue is also often cringeworthy, even more than usual IMO because of the intended seriousness (as opposed to many FF characters who are almost portrayed as caricatures of themselves).