Are you talking about the in-game treasure vault, or the forum category named "Treasure vault"?

In-game: It is a bonus level in Spiral Knights that has very little monsters, and large amounts of money and resources. Getting here takes either luck, or timing the elevator cycles.

Forum category: This place is for Spiral Knights literature, so to speak. It's all about stories, art, etc. Anything along those lines that draw inspiration from the game.

Hope that helped you out :)