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 often considered non-canon, though this might be a bit of short sightedness as it is possible that it had the additional purpose as a monitor for the Vault Experiment, and the only Vault that was actually designed to safeguard citizens.

Vault 0 was constructed by expanding and reinforcing the Cheyenne Mountain Complex in Colorado, and was by far the Largest of the underground Vaults, if the interior spaces were so designed, it could easily support a population of well over a hundred thousand, with whole areas of Vault 0 the size of large hangars, warehouses and factories, these zones were for the construction of various machines that were designed to serve the Vault Dwellers and the Enclave.

It was a place where some geniuses of the pre-war United States could be kept in a cryogenically frozen state. They were hooked up together to one big machine called the Calculator, who was supposed to function together with those minds to build an ideal society in a post-war country by educating the survivors. Due to budget cutbacks by the Defense Department (because of a false sense of safety), several important backup systems were not included to the neuro-link systems, and now instead of releasing the robots to make it safe for humanity, they use the arsenal from Vault 0 and any where else that sufficient arms could be found, to exterminate all life. As for the unfortunate geniuses plugged in, most of them suffered severe brain damage and mental retardation.

Vault 0 thus becomes the end-setting of Fallout Tactics as the player character and his team 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 Explanation for Inconsistency
Due to the nature of Vault 0, it is never stated but could very well have been at least one of the means to observe the Vault Experiment. Vault 0 was said to be the Nucleus of the Vault Network, but when the bombs dropped, communication between Vaults ceased, suggesting that the Vaults were given observational and communication equipment, and Vault 0 blocked out other Vaults from accessing each other in order to keep the vaults isolated, while Vault 0 alone would be able to access the other Vaults' recording systems and databases for observation and review.

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.