Talk to the guard to be able to join Vault City

Go to Vault City and talk to the guard to the right of the inner city gate. Depending on your stats and skills he will require different things of you in order to join Vault City as a citizen. This will make you a faction member of the Vault City NPC faction. However, you won't be able to join Vault City as a citizen if you're already a member of another faction. This includes player factions or other NPC factions. So if you're already a member of another faction somewhere else, you'll need to resign there before you can join here.

• If your Charisma is higher than 8, you can beg the guard to be let in successfully.
• Otherwise he may give you a quest to retrieve the Hydroelectric Magnetosphere Regulator from the reactor in Gecko to gain citizenship.
• If those options are not available, you can gain access if you have any of these:
  • Repair 120
  • First Aid 120
  • Doctor 100
  • Science 150 (capped at 125 so this needs some attention)

NOTE: If you want to do the quest, "collect rent for the guard", then do it before joining the Vault City faction, because it is not available to Vault City citizens.

Rewards

• Joining the Vault City faction (Access to the inner city and Vault 8). You are now a Vault City citizen !