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, 101 (though you will always already know where this vault is), 106, and 108 are. This is helpful in the Optional portion of the quest Agatha's Song.

Enemies
The only resistance inside the building are the leftover security robots and a few turrets. The models inside are regular Protectrons, Sentry Bots, Robobrains and Mister Gutsys. 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 off the Masterbrain, and all other security bots in the Headquarters, if you use the correct command on three employee terminals in the Mainframe area.

Super Mutants have also been found fighting the robots in the Guest Relations area. It seems they have been using the Vault-Tec HQ as a massing point, as Gore Bags which are a common find within Super Mutant bases are located in the Guest Relations mens restrooms.

Guest Relations

 * A Tales of a Junktown Jerky Vendor book on a table up the broken stairs on the right side of the room, in the cafeteria area. It is only accessible when you go down from Administrations again. (Except maybe with tricky jumping.) (Can be done if you take a step back from the edge on the broken staircase and use the extra height)

Corporate Offices

 * Big Book of Science on the top shelf, laying on its side, on two isolated wooden shelves on east wall in a room with almost no floor, second floor northeast corner. You have to crawl out on the remaining ledge of the floor along the wall and look up to grab it.  It's difficult but not impossible to target as it is mixed in with other ruined books, and you may not see it at first. It's in the room due east through a hole in the the wall of the room with the Nuka Cola Quantum referenced below.  See the screenshot in the discussion.
 * Stealth Boy; northwest corner, top floor, in a storage closet just above the room with a random loot safe that also sometimes contains a Stealth Boy.
 * A Pre-War Book on top of a garbage can next to aforementioned safe.
 * Nuka-Cola Quantum on the bottom shelf of a bookcase (second floor of the Vault-Tec Corporate Offices in the room next to one with the Science book). There's a door covering the hole in the floor in the Quantum cola room.
 * There should also be a Scoped .44 Magnum hidden in the cistern of the second toilet from the end in the second floor ladies' room near the exit to Administration. Same floor as the Quantum cola and Big Book of Science.

Administration

 * An issue of Grognak the Barbarian on a curved desk in a private office. The room is at the very west of the area.

Related Quests

 * Agatha's Song

Trivia
Over the front desk right when you enter the building, suspended from the ceiling is Vault 101's door, as an example of Vault-Tec's technology.

On the Administration level there is an employee computer in which you can order Vault-Tec accessories. Upon selection of an item, the terminal will say that the item's cost has been deducted from the employee's salary. It is unlikely that you can acquire those items. When you select "Vault-Tec Bobblehead", it outputs: "Only Available to Executive Level Employee." While this may lead you to believe there is a Bobblehead located in the building, there is not.

Easy Access to Top Level
It is possible to jump from the broken stairs of the Guest Relations Room to bypass the entire second floor. Just position yourself (preferably in third person) on the highest point of the stairs, and you should be able to do it in a few tries. Then, simply take the door to your right and go up the stairs. However, be aware that you will end up right near Masterbrain, so if you were intending to use the terminals to shut him down you may have a more difficult time.

Activating the Vault-Tec Mainframe without opening the Mainframe Door
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 can activate the the Mainframe terminal from below, by jumping from the corner pipes to the right as you enter the room. From the pipes, jump on to any of the the big computers, and walk up as close to the Mainframe as possible. The option to interact with it should appear.

Master Brain
Destroying Master Brain prior to accessing the three employee terminals and examining Masterbrain and choosing to turn off all others can have the result of the Xbox 360 to hang.

Master Brain
Exploit Bug: After turning off the Masterbrain, a lot of the robots on the lower levels (even the dead ones) will regain 1 hp and you can kill and loot them anew.