I never played the evil route myself, but this is how i did it. I always went for the redemption arc with Shadowheart being a Selunite in the end. So i kept Nightsong alive, because she's a powerful ally against Ketheric and the Absolute Army in the end. Also she beats up Lorroakan pretty good. So my CG Bardlock also told him that Nightsong is dead, which in that case was a lie, though. Then i told her about him looking for her, then i went there with the Nightsong/Dame Alyn to kill him together. Was very satisfying. In the end i could plunder the Vault easily.
Anyway, in your situation, i would try to speak to the Librarian in the back of the reception area. After passing some Persuation checks, she may tell you, that there's a key to the Vault in her office - which is upstairs. Some people successfully already broke in to that office and just stole the key. There's several videos on Youtube about that. Maybe that is an option to try for you?
