alright you have to disarm the bomb on the helicopter by either searching the helicopter after it lands, or stealing the detonator from the engineer that is up there waiting with you. Second, the sniper is up on the sniper tower right behind the presidents stage. Climb the tower and take him out, not the other NCR guy. If you shoot the sniper from a distance you will be shot by the NCR.
If this is all too complex, wait till the president gets out, and simply fire your gun into the air. You lose a bit of fame but the quest completes.
1. A bomb on the Vertibird 2. A sniper on the tower (he pushes the original sniper off and takes his place) 3. An engineer with a detonator to ... something.
Disarm the bomb, kill the sniper or pick pocket the engineer and tell the guy who's in charge of security about it and he'll end the speech and the president will escape. ...although I suppose if you don't disarm the bomb he'll die anyway.
Or you could disarm the bomb, kill the sniper and then pick the detonator and tell the guy about it after the president has finished the speech. He'll send rangers to take the engineer.
A bomb? Really? I killed only the sniper guy, the enginner was shot by the security and the president survived anyway. So i guess to find the bomb is not really necessary, only the sniper is important. Or is just got lucky! :-D