The Vault-Tec Corporation was a company contracted by the United States government before the Great War to design and produce the vault system, a vast network of complex bomb and research shelters.
Vault-Tec designed and constructed advanced technologies. It was a major military contractor before the Great War, and as such, developed highly experimental, advanced and secret technologies. In time, Vault-Tec became so thoroughly integrated with top-secret military research done by the Pentagon that it had practically become a department of the U.S. government. The company still remained in private hands, but because of its strong connection to the American government, the principal personnel of Vault-Tec - its executives, top scientists, engineers, and so on - had to be aware of the true purpose of the vaults and Project Safehouse.
Regardless of the American government's directives, Vault-Tec's high-ranking personnel had no intention of submitting to a lottery and living underground with a bunch of human cattle in the intentionally defective vault facilities that they had constructed. Vault-Tec instead created their own private emergency shelter facility for their top employees and their families, removed from the larger vault network, and kept secret from the U.S. government. This facility is now known as the Secret Vault. Vault-Tec did not stop there. This secret, private installation was used to research new, more advanced technologies that would benefit mankind in the midst of post-nuclear environment. This initiative included some extensive research with the "Forced Evolutionary Virus" (FEV), the results of which, unlike the government-controlled research on the virus' uses at Vault 87, was also kept secret from the government. It is known that the FEV stored in the Secret Vault was highly modified in an attempt to achieve its original goals and also avoid sterility among those subjects infected and mutated by it. However, this modification proved unsuccessful.
Other Vault-Tec projects that were intended for the corporation's own use included the development of their own robotic systems, a special biological weapons program (the results of which were probably the creation of a unique race of radroaches and a variation of the deathclaw), advanced biomedical research on the effects of rapid cell regeneration, and even an attempt to create a model of power armor to be used by Vault-Tec personnel.