This is the place under Lovis's tower, right? This post is in the wrong forum, but here's how you get it:
There are two devices in the vault. Activate them to make platforms appear to a switch. Press the switch and you'll see chains lower with four weapons. Don't take them. They're not that good. Instead, investigate the corners of the room. Behind a pillar should be a skill book on a chain. That's the real prize. Take it.
To leave the vault, just go out the door.
