I can see the Vai gathering together a few generations later. As they network in their secret meeting place (next door to the diner with the "two silver discount on meals to all Vai members" in the window) they come to a momentous decision. Namely, it's a hassle killing people. Besides, it isn't strictly necessary. Ants, rats, ambulance chasers, they're all good. And you don't have the trouble you get when you kill a sapient creature. (In the case of ambulance chasers you may even get a reward.)
So the decision is made, and soon their old meeting hall has a new sign, Vai Exterminators: Bugs, Bats, and Bunnies eliminated (ask about our ankheg rates). Not only are they fulfilling the terms of their agreement, they're making money on the deal.