Vaan wasn't the leader, Ashe was. she just got guidance from the others.

basch: basically just kissed her ass the whol way thru
and Vaan and penelo didn't have a clue.
Fran and Balthier had the know how and adviced her against the nethicite (reddas as well) and Fran seemed to have the navigating down.
so without them Ashe would have been screwed.