Collect all three musical compositions from the front computer in the main lobby and play them near the speaker in Vera's suite to unlock the elevator. Alternatively, Christine can say the password in her new voice.
Discover the secrets of the Sierra Madre at the Vault's central chamber.
(Optional) Sneak out of the Vault's chamber area undetected, trapping Elijah when he opens the vault.
Escape the Sierra Madre Vault before your collar explodes.
Behind the scenes
If the player chooses to read Sinclair's personal accounts, he/she will find a letter to Dean Domino. At the end of the letter it says, "...and I will raise my glass and say 'Fortunato.'" This is a reference to Edgar Allen Poe's "The Cask of Amontillado", in which the protagonist Montresor lures his victim, Fortunato, to an underground crypt in his family catacombs, where he chains him to a wall and seals him up alive in an alcove. This is similar to how Sinclair was supposed to have trapped Dean Domino in his own grave at the bottom of the Sierra Madre. It is signed "in pace requiescat," Latin for "may he rest in peace," the phrase with which Poe's story ends.
PCPlayStation 3Xbox 360 When trying to return to the Mojave Wasteland, the game can freeze on a black screen forcing you to reset your system.
A possible fix to the above bug (on hardcore mode) is making sure as you're leaving to satisfy the couriers hunger, thirst, sleep deprivation, and radiation. (confirmed)
PlayStation 3Xbox 360 There is a glitch where dying after speaking to Elijah but before reaching the elevator can cause you to reload an autosave in front of the vault monitor, but the second it loads you die. This causes a reload of the same autosave, which creates an infinite loop of dying and reloading. Saving before speaking to Elijah should fix the problem.
PC Sometimes, if you disable the turrets before Elijah comes into the vault, he remains behind the force field, and you cannot kill him, thus preventing the add-on's completion. He may remain behind the force field and shoot at you with his Gauss rifle, which still injures the player. You cannot damage him through the field.
PC If you sneak past Elijah and get to the elevator safely through the ducts, the game will crash to the desktop, losing your achievement. To fix this, you must restart the game, and load your last save.
PC After killing Elijah, the elevator may take you to Vera's suite, rather than the Villa's Fountain. If this happens the quest will not advance as it should. At this point if you try to re-enter the vault, or leave the casino your collar will explode. However, if you use the console commands tgm and unlock on the casino's doors, you will be taken to the fountain. Head to the Mojave Wasteland gate, and change your view to third person. Then, open the console and type showinventory, then select the player. If you now cannot see the collar id on the console, drop items until it is visible. Remove the collar using player.removeitem X # (X=item id #=amount of item). With the collar removed, pick up your gear and use unlock to open the Wasteland gate. This will allow the ending slides to play, and you will be taken back to The Abandoned BoS bunker. When back to the bunker, you can unlock Elijah's terminal, Elijah's footlocker, and the footlocker beside the ladder with the unlock command. Console commands can be used to complete the quest afterwards. You will then be able to continue the game, still gaining the perks and items you would have if this bug had not occurred.
PC Sometimes after you kill Elijah the force fields will remain up, trapping the Courier in the vault until the explosive collar detonates.
PC After killing Elijah the quest marker will not update. The marker would point to his body even after it said to escape. Prior to removing the collar with commands and leaving the Madre, approaching the exit would play Elijah's warning about leaving without doing what you were told. After playing the slideshow and arriving back in the bunker, the marker for the quest remained pointing towards the room with the speaker playing the broadcast.
PlayStation 3 If you are over-encumbered upon using the terminal in the back of the vault, speaking with Father Elijah will cause the dialogue to auto-skip. This bug continues to occur even if you reload from the terminal auto-save, and are not over-encumbered.
Xbox 360 When opening the computer in the Vault, after completing the mission and the slide show you will be locked in the vault, unable to get out.
Xbox 360 The maintenance door is still locked and closed, even if opened (as if there is an invisible wall present), effectively trapping the player until their collar blows up. The best thing to do is wait it out for a few moments and it will then let you walk through it.