When you start the quest, you're told that Nick was last seen near Park Street Station, which is located at the northeast edge of Boston Common close to Goodneighbor. Leave Diamond City and head east by whichever path you choose to take. Be wary though, the area is full of many different kinds of enemies. You can encounter Raiders, Scavengers, dogs and Super Mutants so have your wits about you, keep plenty of ammo on hand and know when to run if you are overwhelmed.
Once inside the station, head down the stairs and you'll encounter 3x Triggermen. Triggermen are a mix of human and non-feral ghouls. They fight with primarily pistols or submachine guns. Fortunately, this area offers lots of cover so take your time and get them down. Explore the area as there are a lot of useful crafting items to be be found.
Once you've cleared the area, head out through the only door and carry on down the stairs to the station platform. Here you'll find 6x Triggermen. This fight becomes a little more hairy as there is a lot more open space for them to spread out and if you're taking hits from multiple submachine guns you won't last long. Bait them to chase you to the stairs and take them out one at a time. Once the group is down, follow the tunnels until you come to the next platform, where another 5 Triggermen will be waiting for you. As before, bait them out to separate them from the group and take them out one at a time. You'll now have reached the door to Vault 114. Interact with the panel to open it and head inside.
In the Vault atrium you'll find three more Triggermen. Once they're down, explore around and read the terminal located here to find out about Vault-Tec's little social experiment for Vault 114. Follow the corridors and head deeper into the Vault. You'll come to a corridor with two Triggermen chatting to each other facing away from you. A grenade or molotov cocktail works particularly well here.
Carrying on, you'll come to an open area with several gantries and 5 Triggermen. As before, bait them out and kill them one by one. Once the area is clear, follow the gantries to the room at the bottom and there'll be a hatch open in the floor. It's a one way trip, so drop down. You'll next come to what looks like a large mess hall/recreation area. Up above you'll see Dino, one of Skinny Malone's guards, taunting Nick Valentine who is locked in the Overseer's office. Kill Dino, then loot his corpse to find the password for the terminal on the wall. Unlock the office and head inside. Talk to Nick and exhaust all the available dialogue. Before you follow him out of the office, make sure to pick up the Speech Bobblehead from the Overseer's desk.
Nick will now lead you out of Vault 114. Deal with the several sets of Triggermen you find however you wish (Nick gives you the option, be quiet or be loud). Following Nick you'll eventually reach the exit where the boss Skinny Malone along with Darla and bodyguards are waiting.. There are several ways to put an end to this conflict, and it is possible to escape without violence or to turn Darla and Skinny Malone against each other.
After the conflict is resolved, Nick will return to his office in Diamond City. Meeting back up with him will let you receive your reward and continue your search for Shaun.
Unlikely Valentine Walkthrough