This Quest still has a lot of Glitches....
So, as far as i can say,
If you are working with Yes Man and do You'll Know It When It Happens quest before doing this one and choose to bring a truce between NCR and the Brotherhood it will bug out and you will not be able to complete this quest.
When speaking with the Colonel she will ask if the President is safe and you will have no dialogue choices to complete the quest even though you already did it.
Also if you've already done that mission you will be stuck at hoover dam waiting for the president and grant will act as if you've already saved him (pretty much messing you up royally)
To prevent this simply do You'll Know It When It Happens directly after killing Mr. House. Then go talk to Moore (You can say now that the president is safe) and choose the "I don't wanna go yet"-option when the dialog about the final battle appears. Afterward, you may talk to Yes Man and start Wild Card: Change in Management and continue his way.
You can still work with Yes Man and bring a truce between the NCR and the Brotherhood during this quest as long as you recieve the quest You'll Know It When It Happens from Colonel Moore and not from Mr. House or Yes Man.
Similarly, completing the You'll Know It When It Happens quest for Mr. House will result in a bug where you can't talk to Grant to start the quest and you can't tell the Colonel that the President is safe.
Oddly enough, after saving the President for Yes Man! it is possible that Colonel Moore and the ambassador despawn.
Hope that helps....
CRUZ :vaultboy: