The Stormwind Vault is a prison within the human capital city of Stormwind City, designed to contain extremely dangerous entities: demons, rogue mages, ogre mages, necromancers, and other nightmares, anyone or anything else that could not be held by manacles or iron bars. Similar in many ways to Tol Barad, the Vault is kept secret from the public.
The Vault is located opposite the Stockade, and mirrors that other prison in most ways, except that the Vault is completely surrounded by water and has a clock on top of it. It is currently inaccessible, with both of its doors gated off by a portcullis and watched by two guards.
Although there were guard NPCs there and the geography apparently matches the entrance to the Stockades, the Vault has never been officially accessible, not even in alpha or beta.
There is, however, a map model (.wmo) in the data files named "Stormwind Prison" (stormwindprison.wmo), which could possibly have been an outline for an endgame dungeon, or possibly even raid. The map is similar to The Stormwind Stockade in style, however there are well over twice the amount of vaults and hallways winding throughout the prison. Several boss rooms also seem to be larger than in the Stockade, and at the bottom of the prison, there includes an extremely large "cell" held up by four pillars, similar to the room seen above the stockade, in the fortress above (Stockade Entrance, part of Stormwind City).