Give Spy Holodisk to authority in NCR
A holodisk about spy reports on NCR
When you are in Vault 15, go to where Darion was standing and scan the computer right behind him. If your science skill is high enough you will get the NCR spy holodisk. Take the holodisk to NCR. If you tell president Tandi about them having a spy within her government, she will realize that Feargus (the person in the Hall of Congress entrance room) is the traitor and tells you to give the disk to her aide Gunther.
Once you give him the disk he will award you with $4000. Afterwards Feargus will promptly leave the republic after having been disgraced or executed for high treason (you can use the holodisk to transfer the data on it to your Pip-Boy 2000).