When the player first enters Beregost in chapter 3, a child sends a message to you to meet Vai in Jovial Juggler. Don't go there unless your reputation is higher than 5 or she will be hostile, rendering the subsequent rewards unobtainable.
Vai will 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 original versions of the game, the player can also sell winter wolf pelts and wyvern heads to her though this is not explicitly stated, as a result, you may prefer to keep her for a little while, at least long enough to complete Coran's quest to kill wyverns and bring wyvern heads to her for 1000 each because she will leave for Baldur's Gate if you talk to her with no bandit scalps in Chapter 4.
In Enhanced Edition however, she only buys bandit scalps.
After the player has cleared out Bandit Camp and advanced to Chapter 4, talk to Vai with no bandit scalps in procession will flag the completion of the Kivan and Tazok companion quest and Kivan to be a potential permanent party member whether you have recruited him or not, the report will also cause her to leave unless there is a bandit scalp in your inventory. Your final reward will be +1 reputation point.