This quest is available in Vault 81. After donating blood to Dr. Forsythe and bringing Erin’s cat back home during the “Here Kitty, Kitty” quest, you’ll get a misc quest to visit Austin in the clinic. You’ll find Bobby, Dr. Penske, and Dr. Forsythe talking to each other. Let them finish their conversation with you around and Dr. Penske will open up this quest. Follow Bobby as he leads you to the secret vault entrance. Enter the vault and start exploring.
Go upstairs and a mole rat will ambush you. Beside the mound where it emerged, you can find a crate containing fragmentation mines. A short walk along the corridor lies a first aid box. Continue to the next room where you can find a working terminal. It only contains logs so read it only if you want to.
Go through the next door, and there’s a Novice toolcase on the floor in front of you. There are also laser turrets on the ceiling along the main corridor so destroy them first before looting that tool case. There’s also a hostile protectron inside the room to the left.
Go through that room and to the next corridor. There should be a Novice crate beside the stairs. Once looted, go downstairs. The cafeteria doesn’t have anything of interest so head outside and destroy the ceiling auto-turret. There’s a protectron you can activate here by hacking the Novice terminal beside it.
Continue down the corridor until you reach the reactor room. Go upstairs and the molerats below will burst out to ambush you. Dispatch them and loot the nearby aid and chems from the containers near the stairs. Continue upstairs and go through the security gate. The observation terminals you’ll find along the way doesn’t do anything special but they provide clues as to what this secret vault is all about.
Continue along the corridor and loot whatever interests you until you reach the next security gate. Go downstairs and head to the next room. Some molerats will burst out from the holding cells and attack you. There’s also a Master wall safe in the room across the holding cells.
Go upstairs and you’ll find an Expert terminal (the password can be found nearby so don’t bother hacking this unless you’re after some extra EXP). This is used to unlock the nearby door to the right. Continue ahead and you’ll encounter the mole rat brood mothers and more mole rats. Kill them all and loot their bodies. One of the fallen cabinets contain a mini-nuke and the steamer trunk nearby contains some ammo as well.
Head to the next door and find the Secret Vault 81 password on the table and a terminal you can access. The fridge beside the table has some free chems and aid while the table on the other table has a stealthboy. Since you have the password, return to the Expert terminal in the same level and use it to open the door.
Continue to the locked room and you’ll encounter a robot named Curie. You can claim that you’re a Vault-Tec employee and authorize her release. Do so and she’ll open the door. She’ll give you the Vault 81 cure. Since there’s only one dose, you’ll be left with two options later on.
Don’t forget to get the Medicine Bobblehead from the table inside Curie’s room as well. Once done, you can now leave through the nearby elevator. Head back to Forsythe’s clinic and either hand him the cure or keep it to yourself.
Choices:
Cure Austin: You’ll get the Syringer weapon, get your very own room in the vault but you’ll have the permanent effects of the Molerat disease (permanent -10 Max HP)
Cure yourself: Austin will die and you won’t get any new weapon or room. The vault residents will blame you for it.
Also at the end of the mission, Curie will offer to join up with you as new partner. Additionally, you’ll be able to loot most of the items inside the vault without “stealing” them.