In the Fallout TV series, it's revealed that Vault-Tec is willing, and even planning, to drop the bombs themselves as they have invested everything in the apocalypse. It has to happen or they will collapse, so they invite some of the biggest corporations in America to a shadowy meeting where they discuss how to bring about the apocalypse from which they will all rise as unstoppable monopolies.
This means that vaults are divided among a bunch of seedy corporations that decide what experiments to run, finally giving us a reason behind the senseless cruelty of these advanced bomb shelters.
The original concept was that the Enclave was using vaults to prepare mankind for generational space travel , explaining the inhumane studies, but this idea was scrapped.
In the meeting, excited at the prospect of controlling the last vestiges of humanity, CEOs from Repconn, RobCo, West Tek and more excitedly throw out different ideas for experiments, all of which reference vaults we've heard about or seen in-game.
Big MT CEO Frederick Sinclair suggests overcrowding a vault so that people would have to compete to survive. It's only ever mentioned in the games, but we know that he stuck to his plan and sent 2,000 people to Vault 27, over double what it could hold.
Repconn discusses using a milk delivery robot to govern the inhabitants of its shelter, which is exactly what happened in Vault 51. This one can actually be found in Fallout 76 in the Appalachia region, where we discover that a supercomputer was tasked with selecting the shelter's leader due to the purposeful lack of an overseer. The AI eventually began creating disaster scenarios to see who would rise to the occasion and prove themselves worthy of leading, while running miniature--but no less humane--experiments to test the populace.
West Tek wanted to create super soldiers, which is what happened in Vault 87. Found in Fallout 3, the inhabitants of this shelter were unknowingly subjected to the FEV virus,
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