I dont have NV, but I do have FO3, and what I can tell you is get your speech skill higher.
Is this one of the quests your on?http://fallout.wikia.com/wiki/Oh_My_Papa
I killed all the Khans them all
During the daytime you will see Regis, Papa Khan and Karl sitting and eating behind a long table.Talk to Papa Khan. Exhaust all chat options until a long sentence asking Papa Khan to abandon the alliance with Caesar's Legion appears. He will refuse regardless.Exhaust all chat options with Regis and Karl.Exit the Longhouse; Regis will speak to you about how to convince Papa Khan to ally with NCR instead of the Legion - obtaining the support of the 4 advisers (Regis, Jack, Diane and Melissa) & discrediting Karl.
Better idea : Talk to the legion dude and do the speech checks to expose him infront of papa khan
Work around for PC version:</b> Open console and type "resetquest 00140c3a" to restart the quest and finish it the way you want. Another workaround for PC version:</b> You can just progress the For the Republic, Part 2 quest by following series of commands: SetObjectiveCompleted 00136166 20 1 Now to advance it either use the following: SetObjectiveDisplayed 00136166 25 1 - Tell Colonel Moore the Khans will assist in upcoming battle. Useful if you have already taken care of Omertas. SetObjectiveDisplayed 00136166 26 1 - Khans move out or are wiped out SetObjectiveDisplayed 00136166 27 1 - Regis takes over, sides with NCR Afterwards the NCR questline will follow without problems.