Vault City
1. Population driven to NCR by radiation: Don't fix the power plant (blowing it up works too).
2. VC independent, Westin and Lynette marry (!): Fix the power plant, kill the raiders, expose Bishop's plan and deliver the two holodisks but do not collect the final reward from Randal (bug), and don't kill Westin or Lynette (obviously).
3. VC annexed by NCR: Fix the power plant, don't bring the Account Book or Bishop's Holodisk to Lynette.
4. VC becomes part of NCR: Fix the power plant, kill the raiders and bring the Account Book or Bishop's Holodisk to Lynette. (You get this one if you fail to get ending 2 for any reason, e.g. killing Westin or picking up the reward.)