When the player first enters the Jovial Juggler in chapter 3, Vai will approach you and introduce herself. She laments that bandits in the area have cut off the roads to her headquarters in Baldur's Gate and asks you to sort out the bandit problem. From that point on, she offers to buy any bandit scalps from the party for 50 gold (in BG:EE she only buys bandit scalps; in other versions of the game the player can also sell winter wolf pelts and wyvern heads to her though this is not explicitly stated). If your Reputation is lower than 5 then she will attack the party instead.
After the player has cleared out Bandit Camp, Chapter 4 will begin and talking to Vai anytime after this will cause her to leave and give you your reward unless you are carrying a bandit scalp. Your reward will simply be one more reputation point.