Los Angeles was the site Vault-Tec chose to build their demonstration vault. It was a fully functional shelter, though its design varied from the one used for later vaults. As a demonstration, it was extremely successful, and jump-started Vault-Tec's Project Safehouse.
During the Great War, people hid in the vault, which functioned as intended: it protected the inhabitants from the hazards of the nuclear holocaust. When it opened years later, the inhabitants that emerged into the Angel's Boneyard eventually went their separate ways. Some formed the Adytum, others the Regulators, while others chose to explore the wastes.
It was the latter group that was found by the Master's scouts, thus giving him the location of the vault. Relocating there, Richard Grey permanently integrated himself into the vault by neurolinking to the central computer. Slowly his body grew to be a part of it as well.
The entrance to the shelter goes through a small cave, linked to the Cathedral's basement. Upon entering, differences from standard vaults are evident: the airlock is much smaller, the EML-2 room is designed with additional windows, and the main hall is one large chamber.
During the Master's reign, two nightkin stood guard at the entrance at all times.
The second level also differed in layout, with the elevator located to the side, not in the center. It is possible the supermutants made alterations to the original design. In 2161, the second level housed a chapel, barracks for supermutant guards, a large research lab where mad scientists worked on awakening the psionic potential in humans, and the psyker holding cell and test subject cells.
The third level housed the heart of the Unity. Here, apart from the most elite nightkin detachment and their leader, Viscious, the Master resided, united with the overseer's command chair. It is here that the elevator to secure storage is located.
The deepest level of the demonstration vault was where the Master kept his ace-in-the-hole weapon. Should the Unity face an enemy it would not be able to defeat, the supermutants would deploy one of the last few remaining nuclear warheads against them. For security reasons, the key to the console was kept in the possession of the supermutant Lieutenant.