In the game "Fallout 3," the vault key opens a small room in Point Lookout that contains some useful items. Inside the reward vault, a pair of first aid kits hang on the wall, while some ammunition and a unique weapon, the Microwave Emitter, rest on a workbench.
Following instructions from IGN, a player can receive the reward key by siding with Desmond in the fight against the Brain and the Point Lookout Lighthouse. The Brain deploys a wave of Protectrons, enemies that attack both the player and Desmond. The player must fight off the Protectrons and keep Desmond alive to claim the vault key. After the fight, Desmond hands over the key. To find the reward vault, the player returns to the hallway outside the Brain's room and goes to the doorway directly across the way. In this room, there are two doors, one in the center and one on the right. Going through the center door leads the player to enter the Reward Vault. There is a ladder in the vault that allows a quick exit from Point Lookout.
After receiving the key from Desmond, he becomes a relatively inconsequential character in the narrative. A player can kill him and loot the body to receive a unique Sunglasses item, or leave him alive. Neither choice significantly affects further plot developments.