But Vaan's not the main character... the development team has even stated that XII really has no actual "leading man".

From a story standpoint. Lady Ashe is the true main character cause the main goals and drama of the game are all about her. Vaan's plot ends once the party reaches Bhujerba and afterwards he's more of an observer until the very end with all the adult characters actually taking center stage.

Even when they were discussing who to put in Dissidia for XII, Vaan was not even a consideration. They were going to go with Balthier instead, despite Basch being the original lead character.

Saying Vaan is the lead in XII is like trying to decide who the main character is in VI and god knows no one likes being in that storm of a discussion...