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, gated off by a portcullis with 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.
The clock at the top of the vault which was stopped at the time of Deathwing's attack on Stormwind City is a reference to the time at which the atomic bomb was dropped on Hiroshima City which was also at 8:15.