This quest can be started by talking to Dr. Madison Li in the Rivet City Science Lab, with or without completing the Galaxy News Radio quest. Also it can be started by picking up the notes in Jefferson Memorial regarding Project Purity, or by entering Vault 112 itself.
Gain entrance to Rivet City
It is highly likely that you have already visited Rivet City, likely as part of The Wasteland Survival Guide or in the course of visiting the southern Metro Stations to reach areas like Museum of American History or the National Archives. In this case, reaching Dr. Li is trivial.
- From there, follow the river (Potomac) south on the west bank, passing Wilhelms Wharf, down to the Arlington Memorial Bridge at the Citadel.
- You can either avoid a few super mutants by swimming over at the Jefferson Memorial, or you can cross the Arlington Memorial Bridge and head south after a brief battle.
- When you have the Jefferson Memorial in sight, head east. Near the Anacostia Crossing metro entrance you will notice a metal construct next to the aircraft carrier ; climb it and use the intercom to get the bridge connected.
You can also cross the D.C. urban area, dodging in and out of metro stations, or follow the east bank of Potomac, and it will lead you to a lot or at least some more combat.
Talk to Doctor Li about Dad and Project Purity
Dr. Li knows your father, James. She offers details on Project Purity and tells you where James ran off to: the Jefferson Memorial. If you've been to this location and visited the underground tunnel, the game will allow you to tell her that he isn't there; she will instead insist that you go back as his notes were left there.
Look for Dad in Project Purity's Control Room
Enter the memorial through the metal door labeled Gift Shop, it's on the right front side with a metal frame around it and a set of lights next to it. Be warned that there are a number of super mutants inside. Make your way to the Rotunda by following the signs.
Once in the Rotunda, walk up to the auxiliary filtration unit near the entry way and pick up the Project Purity Journals resting here. One of the tapes, #10, will indicate that your father has left for Vault 112 already. The location will be added to your map; there are several other points of interest in the area, which makes it relatively simple to fast travel to.
Gain access to Vault 112
Walk or fast travel to Smith Casey's Garage Once inside progress past the front desk and into the back room, killing the radroaches and mole rats. You will find a switch located on the wall to your left that operates a large hole in the floor. Activate the switch and the door will open. Proceed down the stairway, down the next set of stairs and to the bottom. You cannot miss the Vault 112 doorway. Activate the door using the panel and you can walk in. Next walk down into the tranquility lab and find the only unoccupied operational tranquility lounger where you will complete the quest. Smith Casey's garage can be accessed straight after the player leaves Vault 101, therefore advancing through a large section of the game's main quest-line.
It is possible to skip major portions of this quest, for example by speaking directly to Dr. Li in Rivet City before visiting Three Dog, or by visiting the Jefferson Memorial directly. In these cases, this achievement will still be rewarded at the same time that the Tranquility Lane achievement is received.
In the Xbox and PS3 versions, if you already have entered in Jefferson Memorial (especially the rotunda) before talking to Dr. Li in Rivet City, the achievement/trophy will not be unlocked once you materialize at Tranquility Lane. Thus this achievement/trophy cannot be obtained out of sequence, with regards to the main quest achievements. In order to rectify this glitch, the player can load a saved game at a point where Jefferson Memorial has not been explored and work through the main quest again.