Vivi's theme is kind of catchy, but a little childish for my tastes. That stands to reason, since he's a child himself. I liked the other, related themes though - like the music played during the attack on Lindblum, which is based on the same 'Black Mage theme' as Vivi's tune.

One of the most touching pieces of music in the game is "Mage versus Mage", from the cargo ship scene when Vivi is watching the dead Black Mages tumble out of the airship.