Once your reputation hits rock bottom, it’s basically impossible to pull it back up. How do you stop being vilified by the NCR?
You don’t stop being vilified by the NCR. Once vilified, a faction will attack you on sight. This makes it impossible to do nice things that will improve your reputation.
Your only chance is to acquire the platinum chip. This will reset your NCR reputation so you can speak with Ambassador Crocker. If you’re vilified afterwards, it’ll be nearly impossible to complete nice deeds for the NCR.
You can not stop being vilified by the NCR
Becoming vilified with a group in Fallout: New Vegas means it’s nearly impossible to repair your reputation.
Once vilified, the following message appears: “For your overwhelmingly monstrous behavior, you have become vilified by the community.” This means that members of that faction will attack you on sight.
Normally, NCR hit-squads will warn you of your falling reputation. If you become vilified before the warning, they will simply attack you on sight without speaking first.
This poses obvious problems. If you’re being attacked on sight, then it is nearly impossible to find opportunities to raise your reputation.
A possible option is playing through the main quest and acquiring the platinum chip. Once acquired, all factions will reset your karma and offer to speak with you.
Should you ignore this opportunity and choose the Legion instead of the NCR, or simply harass NCR soldiers, your reputation will plummet again.
If you’re vilified with the NCR, your best option is to get the chip from Benny and reset the reputation. If you’ve become vilified after already receiving the meeting-offer from Ambassador Crocker, then it’s too late.
So that’s the answer. You can’t stop being vilified by the NCR. Vilified means they’ll attack on sight, so you can’t complete good deeds for them.