Vault-Tec Headquarters

The Vault-Tec Headquarters building is located in the Vernon Square area of the Washington, DC ruins.

This was Vault-Tec's base of operations around the DC area (it is unknown if this was their main headquarters). From this location (specifically, from the Vault-Tec Mainframe), you can learn where Vaults 87, 92, 106, 108 and 112 are.

Enemies
The only resistance inside the building are the leftover security robots and a few turrets. The models inside are: Protectron, Sentry Bot and Mister Gutsy. Also, there is one special model of the Robobrain on the level where the Mainframe is. It is called the Masterbrain and is hostile towards you. You can turn it off (and every other security bot in the facility) if you access three terminals that are placed around that area.

Trivia
Over the front desk right when you enter the building, suspended from the ceiling is Vault 101's door. It is unknown why exactly 101's door is there.

Bugs
Some people have reported that getting to open the doors to the Vault-Tec Mainframe does not work for them, i.e. selecting the proper command on all of the three terminals still does nothing to the doors, they remain shut. There is an alternative way to reach the Mainframe. The player needs to jump on the pipes that are found around the computer room below the Mainframe access. From the pipes, jump on to the big computers, and walk up as close to the Mainframe as possible. The option to interact with it should appear.