The rumours are indeed true and Vaan is the least plot-important playable character. So far that's established. However, Vaan fills a vital role in the party that I can't remember any other character in any Final Fantasy I've played (i.e. 1-10) filling so obviously. Vaan is the official party bitch.
I can just imagine a usual party meeting in the back room of a Rabanastre tavern:
Basch: Vayne gathers his forces. We must hurry and find a way to stop him, else all is lost.
Ashe: Let us leave at once for the mystical temple and seek the mystical stone.
Balthier: Good thinking, but we should go along the coast. The empire's troops won't be looking for us there.
Vaan: Coast? We're going to the beach? Awesome, I'm packing my swimming trunks!
Silence.
Balthier: Vaan. Vaan, why don't you go out and buy us a few new suits of armour? A few hundred potions and remedies, too. Be so kind, would you? Oh, and pick up my shirts from the laundromat while you're out, there's a dear.
At which point you, the player, take over and guide Vaan through an exciting city adventure full of calculating prizes and haggling with merchants. This is the reason you only ever play as Vaan inside cities.
At least it's a change from the usual "The main villain grew him in a vat."