twillight: In theory NCR's ending depends on wether they made pact with Vault 15, and wether Westin is alive,and finally wether Carlson is alive.
The ending here you probably looking after is where yu don't kill Westin (neitherCarlson), but don't make a deal between NCRand V15.
Vault City will be eradicated if you don't fix Gecko.
Vault City will be annected by NCR if you don't bring the Account Book or Bishop's Holodisc to Lynette.
Vault City becomes part of NCR if you fix the power plant, kill the raiders,bring the Account book OR Bishop's Holodisc toLynette AND don't get VC becoming independent.
To make VC independent it'd require the same stuff as becoming part of NCR, with the exception of Lynette OR Westin being dead - AND
in the not-restorationed game youdon't gain the final reward from Randal.
I don't know if the restoration project patched the above bug, but seems to be safe if you just assassinate Westin/Lynette. The old "I pickpocket you a bomb", or "I heal you until you die" trick should work, and in the restoration project you can even [url= assassinate him properly] http://fallout.wikia.com/wiki/Assassinate_Westin_in_NCR_without_making_it_look_like_a_murder[/url].
I believe NCR becomes independent if you: fix and improve the power plant, kill the raiders and bring the holodisk to Lynette (and follow her instructions), then ensure Westin lives.
I believe if Westin dies (irrespective of other choices), then Vault City joins the NCR. So if you don't want them to be independent then take him down (there's a mission for this from Bishop which I assume you already know, plus another one against NCR after that).
The V15 deal mentioned is the biggest influence on NCR - if you complete it, they gain power and expand - basically in the mostly-benevolent way backed up by military power as per canon. However, if you don't complete it, then what happens with NCR is dependent on whether Westin is alive, Carlson is alive, or both are dead. Not completing the V15 deal is bad for NCR no matter what, but it seems that the more NCR politicians you remove, the worse their society becomes...
