Vault 0

Vault 0 (Vault Zero), unlike most Vaults involved in the Vault Experiment, was not an experiment in human behavior. For that reason, Vault 0 is largely considered non-canon to the lore of the Fallout series. Considering the possibility that Vault 0 had the additional purpose as a monitor for each and every one of the other Vault Experiments, and one of the only Vaults that was actually designed to safeguard citizens, along with control vaults such as Vault 8, this might be a case of short-sightedness.

Vault 0 was constructed by expanding and reinforcing the Cheyenne Mountain Complex in Colorado and was by far the largest of the underground Vaults (by volume). If the interiors were so designed, Vault 0 could easily support a population of well over 100,000 people. Many of the areas that comprise the massive interior layout of Vault 0 were originally designed to be the size of (and were often compared to) aircraft hangars, warehouses and factories; these zones were for the construction of various machines that were ostensibly designed to serve the Vault Dwellers and the Enclave.

Vault 0 was a place where the geniuses of the Pre-War United States could be kept in cryogenic stasis - their brains were extracted and frozen for the duration of their "residence" in this Vault. They were hooked up together to one big supercomputer called the Calculator, which was supposed to function in collaboration with the brains of these Pre-War geniuses to design and nurture an ideal human society in a post-war U.S. by educating the survivors and residents. One should particularly note that this project goal differs from that of the Enclave in methodology.

Due to budget cutbacks by the Department of Defense (because of a false sense of safety as a result of the repeated drills), several important backup systems were not included to the neuro-link systems; instead of releasing the robots to make the Wasteland safe for humanity, the robots manufactured from Vault 0 used its arsenal and anywhere else that sufficient armament could be found, to exterminate all life. As for the unfortunate geniuses plugged in, most of them suffered severe brain damage, dementia, and mental retardation from a combination of age and prolonged radiation exposure (not mutation). Technology, it seemed, has not been affected by the ravages of nuclear radiation.

Vault 0 thus becomes the end-setting of Fallout Tactics, as the player character and his team of Midwestern Brotherhood of Steel paladins have to stop the machine.

Appearances in games
Vault 0 appears only in Fallout Tactics.

Inconsistencies

 * The concept of Vault 0 is often considered inconsistent with the Vault Experiment from Fallout 1 and 2.
 * The technology, aesthetic and design of Vault 0 have nothing in common with other Vaults.

Possible Explanations for Inconsistency

 * Due to the nature of Vault 0's interior design and infrastructure, it could very well have been at least one of the means that either Vault-Tec or the Enclave used to scientifically observe the Vault Experiment in progress with minimal interference with each of the Vaults.
 * Vault 0 was said to be the "Nucleus of the Vault Network" - but when the bombs dropped, communication between Vaults ceased. This suggests that each of the Vaults may have been given rudimentary positioning and communication equipment for this purpose and that Vault 0 blocked out other Vaults from accessing each other to keep the vaults isolated.  Vault 0 alone, in theory, would be able to access each of the the other Vaults' systems and databases for observation and review.
 * One can consider that Vault 0 could have been a "Control Vault" by design as well, if not by intent by the minds of Vault-Tec. In order to effectively study and monitor the other Vaults, a Vault in which nothing is done to manipulate the outcome (or circumstances) must be used as a standard of comparison.  In Fallout 3, the existence of several other "control" Vaults has been confirmed.
 * Another explanation that would make Vault 0 consistent with the Vault Experiment would be if the Enclave designed Vault 0 to fail as uniquely as it did so that they [The Enclave] could get rid of intellectual opponents and/or would have a large source of military hardware and manufacturing capacity on the mainland if they ever needed it. However, there is nothing to suggest this possibility in-game, except perhaps for the nearly-inexplicable comic ineptness of the people chosen to be the Vault 0 Budget Committee.
 * More likely, Vault 0 was not part of the Vault Experiment by intent, and merely used the name to be more publicly identifiable. Logs found in FO:T suggest that Vault 0 was built by lowest-bid government contractors and not specifically Vault-Tec itself, and the integrity of the project quickly took a backseat to political grandstanding and pork-barrel waste.
 * If the President, his Cabinet, and various unnamed power brokers formed the Enclave to withstand the Great War, then Vault 0 was the project formed by the legislators and officials who weren't important, powerful, or intelligent enough to be included in the Enclave.
 * The utter and catastrophic failure of Vault 0 to do anything it was designed to do can be viewed as a satire of governmental waste and incompetence to be contrasted with the plotting, sinister Enclave - who clearly were right in thinking they wanted nothing to do with this bloated, doomed project or the political wonks in charge of it.

Trivia
Cheyenne Mountain Complex is also the location of the fictional Military Strategic Defense Initiative, like Sentient Computer System called "SkyNET" from the Terminator series of films. Many further Terminator references are made throughout the Fallout Tactics. It is also a real military site; there is also a zoo on the other side of the mountain in real life.

The Complex was also used in the television series 'Stargate SG-1' as the base of operations for the fictional Stargate program.