I noticed that, not just Vayne, but many characters never got much of a chance to show/develop personalities. Take Vann, for example; he is, supposedly, the main character. However, after the very beginning of the game, one could automatically tell that he wasn't really going to be all that important of a character. The same thing happened to Penelo, and even Basch, after a while.
So Vayne wasn't that much different. Doctor Cid got way more time to shine as a villain than Vayne.
(And, in my opinion, the game was pretty much centered around Balthier and Ashe. It took a while, but eventually they essentially turned into the "main," main heroes.)
((Now stop me before i go off-topic... XP))