Basically it’s one of the few vaults that didn’t have a malicious sociological experiment behind it. The extent of the experiment was “make sure all the inhabitants are the best doctors and medical experts, so they can pool their efforts together to help find cures for diseases.” This goes as far as schools only teaching basic everyday school stuff, and medical studies, so children can only grow up with a degree in medicine. To help them do their research, they have an AI computer of some kind (never ended up naming it) that keeps track of all of the data they’ve compiled, and monitors all the inhabitants of the vault.

Everything’s going fine for the vault inhabitants for a number of decades, until the AI suffers a logic error of sorts. It decides that instead of trying to eradicate the diseases and viruses themselves, they should be eliminating the methods through which these viruses spread themselves around. It just so happens that the virus they’re researching at the time is an airborne one, so it shuts down all air ventilation in the vault, depriving all the inhabitants of oxygen and killing everyone.

It deems this to be a success, and basically builds upon that logic by thinking “viruses need organic creatures to spread around, so the answer must be kill all organic creatures.” Thus, if you were to encounter the vault, the AI would be doing whatever it can to kill you while you’re inside.

There’s a thematically similar vault in Fallout 4 the deals with cures to diseases and such, and it’s probably better written in regards to Vault-Tec’s intentionally dangerous experiments, but they’re just different enough for me to be okay with that.