Officially, the vaults were nuclear shelters designed to protect the American population from nuclear holocaust. However, with a population of almost 400 million by 2077, the U.S. would need nearly 400,000 vaults the size of Vault 13, while Vault-Tec was commissioned to build only 122 such vaults. The government, and Vault-Tec, never really believed an actual nuclear war would occur; the real reason for the existence of these vaults was to run social experiments on pre-selected segments of the population to see how they react to the stresses of isolation and how successfully they recolonize Earth after the vault opens.
The total number of vaults is a government secret and has been lost; there were the aforementioned "public" vaults, which numbered 122 and an undisclosed number of "private" vaults. Information on whether Vault-Tec was an international corporation or strictly U.S. based, cannot be released due to Vault-Tec and United States federal regulations.[7] That said, Vault-Tec seems to have constructed some vaults in Canada. In a letter sent to a rejected DC citizen the company offered to provide a list of "Vault-Tec facilities with available accommodations, in exciting locales such as Oklahoma and newly-annexed Canada."
Of the 122 known public vaults, only 17 were control, meaning that only 17 were made to public expectations. All others were designed to include a social experiment, sometimes with a select few of the inhabitants observing the occupants.
While some vaults had 'noble' goals, such as to eradicate disease (Vault 81) or improve the human genome (Vault 75), they had incredibly unethical methods of doing so, often exposing their inhabitants - often fatally - to some danger specific to certain vaults to research the effects.
The few vaults that survived intact for more than 80 years came to serve another, unanticipated purpose: they were an excellent source of pure human stock, uncontaminated by the mutated airborne strain of FEV and prime candidates for conversion into super mutants.
The true nature of the vaults epitomized the insidious nature of the government at its most callous, a running theme in Fallout that the American government was disinterested in the well being of its people even in absolute crisis. The innumerable loss of life caused by making so few vaults, and their intended use as social experiments, and toying with what little remained of the American population highlights this. Worse, if the Enclave met setbacks, failed or were rendered incapable of recolonizing the world, it appears there was no backup plan to utilize the vaults to replenish humanity.