I'm sure Vivi is referred to as "he" in the game. Besides, Zidane keeps telling him to go hit on girls.

Edit - I added some quotes from the game. *POSSIBLE SPOILERS!*


ZIDANE (after talking with Vivi in the Evil Forest): "Geez... He [Vivi] was pretty down... Maybe I shouldn't have made that promise. I don't even know if I'll be able to find her [Garnet]..."

STEINER: "It may be difficult with just the two of us. We should seek Master Vivi's help as well."
ZIDANE: "Why are you calling him 'Master'?"
STEINER: "You fool. That black mage has unimaginable powers... I don't want to get him involved, but alas, it can't be helped. We need Master Vivi's powers to rescue the princess."