When in Vault 15, go to where Darion was standing and scan the computer right behind him. With a high enough Science skill, one can get the NCR spy holodisk (the holodisk's data can be copied onto the Pip-Boy 2000). Take the holodisk to NCR, tell president Tandi about Khans 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 the Chosen One to show the disk to her aide Gunther.
Once the disk has been handed to Gunther, he will reward you with $4000. At the same time, Feargus will promptly head to the exit of the map, mumbling about something being "not good" and "having work to do". His ultimate fate is unknown.
You were supposed to first talk to Feargus about the matter, as the door to the Congress main hall locks itself upon entering the map. If you tell him about the spy, he'll wander off (same as described above) and not let you in. But, if the door was already open from your previous visit, it's not closed and instead locked in open position, letting you bypass this logic.[verified]
If you are also seeing Gunther about the reward for the Retrieve Parts/Gain Access to Vault 15 quest, you'll need to speak to him again to get the relevant dialogue option. This is due to the script compiler having ignored a certain syntactic construct.[verified]