Ah, Vault 81. Possibly one of the few Vault-Tec Vaults still operational after 200 years without being overrun by mutant plants (Vault 22), crazy clones (Vault 108), or psycho musicians (Vault 92). For those not in the know, Vault 81 was designed to help find a cure for every known disease in the human race, mostly using modified mole rats seen in the quest Hole in the Wall. The Vault-Tec twist, however: they were going to use human subjects, infecting them with diseases in order to find the universal cure (it turns out that Vault 81 was actually 2 Vaults in one; one for the general population, and another for the scientists).
But, when the bombs fell on October 23, 2077, only 3 scientists turned up to the Vault. The rest weren't notified of the impending doom in time, and didn't make it. Maybe Vault-Tec shouldn't have hired a Overseer with a conscience. But then again, that's probably the only thing that stopped the Vault from falling into chaos.
Now, for 200 years, this secret Vault had gone undiscovered by the majority of Vault 81's general population, except for the Vault's junkie, Bobby De Luca, where he keeps his secret drug stash. When this is found by Austin (the kid who offers you a tour of the Vault when you first arrive), he gets bitten by one of the infected mole rats, and subsequently gets ill.
So, 24 hours after you leave the Vault (either because you're trying to find Ashes, Erin's kitten, or you need to find all of Eddie Winter's holotapes, or something) and you return, people start talking about how Austin ended up in Dr. Jacob Forsythe's med-bay. After a quick conversation with Austin's "Gran", Bobby leads you to the entrance of the secret Vault.
After a while, after battling through turrets, Protectrons and infected mole rats (if you or your companion gets bitten, you permanently lose 10 HP), you reach Curie, a Miss Nanny assigned to the research project. Her 3 human companions are dead, cremated and "buried" in lockers. After letting you out, she gives you the only cure to the mole rat disease, and it's there you can find the Medicine Bobblehead (Stimpaks heal you 10% more, up to 108% with a full rank Medic perk).
After finding your way back to the med-bay, you have 2 choices (or 3, but the third one doesn't really work well): give the cure to Austin, get a free Syringer Rifle, a room in Vault 81, but be stuck with the -10HP forever; or use the cure on yourself, let Austin die, get attacked by everyone in the place, and get permanently kicked out of Vault 81.
Well, let's just say that I prefer to save kids lives, even if I lose a little bit of health. Goodbye.