How to unlock: Complete the Here Kitty, Kitty quest (M6,3).
Go to the healing facility after you finish the short quest for Erin. Wait for the quarrel between the inhabitants to end, then follow Bobby to the hidden location in the vault. The road is lineal and you must reach its end. On the first floor you will be attacked by cannons. Wait a moment after lurking out - you will be attacked by a Protectron robot.
Remember that you can collect tools for Calvin from the vault. He will pay you well for them and here you can find quite some number of them.
On the next level you will have to fight many Mole Rats. Some of them will be in the "improved version", so don't ignore them. On the floor, just behind entering them using the stairs, you will see a terminal that opens the closed door on the right side. You don't need to hack them, at the end of the corridor you will find a room with a tape lying on the desk next to the terminal. Put it in the door terminal and you will enter inside.
You will find another closed door. Talk with Curie the robot through the glass and say that you're part of the Vault-Tec crew and the door will open. The cure which you are looking for will be given to you by the robot. In the room there's a Vault-Tec medicine bobblehead on the desk. Now, leave the room and drive up the elevator. Give the cure and the quest will end.
Quest rewards: large number of experience points, the trust of the vault (you can pick up most of the items without stealing), your own room in the crypt, Short Syringer Rifle, companion - Curie the robot, Dependency quest.