Even if he is characterised well at some point, it's not relevant to the actual story. Vaan doesn't fit. His individual bildungsroman is not what we wanted and not what we were by and large given; I think of Vaan as I think of the Player Character in White Knight Chronicles: he's your avatar and is only there to be your viewpoint character.

Tidus might harp on about it being his story, and he might be wrong and it is Yuna's story, or all of their stories, but the point is that he's still relevant, and he still undergoes subtle and obvious changes, and there is actual agency from the character. Final Fantasy X is about the party going on a pilgrimage and stopping Sin. Final Fantasy XII is about... something? About the Occurria versus Vayne Solidor? It's not about the party, not really, as Vaan and Penelo are irrelevant, and Fran is only relevant because she has ties to Balthier.