After getting 'resolve the situation in the vault' from Valera, I slaughtered all the Bardattos on the way to the vault to rescue the thief. I then rescued the thief, went back to Valera, and he told me to eliminate the Bardattos, which I've already done. But the game hasn't registered that they're already dead, so I can't complete the quest :/
Great game though, I'm loving it *thumbs up*