@br54
You see there's a pool behind kemm's mansion, you need to switch the ladder to drown the water (the ladder is on the north side of the pool beside the water pump, it'll blink yellow). But before go to the vault, you need to go to the right side of the mansion to steal kemm's wife's ring (either kill her or sneak and steal from her) and equip it. So vault's guardian (lucian) can't attack you after you enter the vault.
