Vault 76

Vault 76 is a vault shown on the computer in the Citadel. However when you go to the map of all the vaults in the Capital Wasteland it is not there. There are locations that are not marked by the map (Rockopolis, Alien Crash Site, and Vault 112) but Vault 76 is not in Fallout 3. Also, it is not on the vault map within the Citadel. This may be a hint at the upcoming DLC.

The Vault-Tec Terminal in the Citadel lists Vault 76 as a "control" vault, with 500 occupants. The vault was designed to open after 20 years. It was among the seventeen total control vaults, meaning it functioned precisely according to occupants' expectations.

It is possible this vault is reserved for future DLC.