Vault 87 can only be accessed through Little Lamplight. At the appropriate time in the game, the player will have the option of questioning mayor MacCready about how to access Vault 87. The mayor will suggest taking Murder Pass. However, he will also inform the player that there is a broken computer terminal in Little Lamplight that will allow direct access to the vault via a different, safer route.
To access the terminal, the player must speak to Joseph, (as directed by the mayor). Joseph can fix the broken terminal, allowing it to be hacked, but he will only do so if the player has rescued Penny from Paradise Falls, or if the player has a high enough science or persuasion skill. Joseph can be found in the Little Lamplight schoolhouse in the morning. The player must have 50 (average) Science skill to actually hack the computer to get into the vault.
If Penny is not rescued, or if Joseph becomes upset through dialogue, entrance through Murder Pass will be the only option to get to Vault 87. Murder Pass houses several super mutants armed with heavy weaponry like missile launchers and miniguns. Murder Pass is a good choice for earning experience, which is north of the souvenir building. Mayor MacCready will open the gate for you if asked.