So... how do super mutants leave and enter Vault 87? Also, how are humans turned into super mutants?
The main entrance is inaccessible, and only the Enclave has the key and the means to survive massive doses of radiation.
Humans also need to be alive to be infected by the FEV. And mutants evidently don't have access to the Enclave tech. So, how do they survive the massive radiation at the entrance?
Both external entries at Little Lamplight are locked down until you arrive, too.