Hi! After a long break I patched New Vegas and started playing again. The Vendor crashes i had are finally fixed (but after buying playing cards it sounds like the sound is looping a few times when the vendor window closes).
I was sent to a village in the south to check whats going on there and the Legion was killing all people there (they handed out lottery tickets). I got a quest to follow them and save the bandits. I found them in a small camp, saved the game and then freed the bandits. The Legion NPCs attacked me and I loaded the previous game where the Legion was still neutral but after loading they immediately attacked me! - the Faction was not reset/loaded from the save. I had to exit to the main menu and load the game to fix it.

In the end I found a solution where I could free the bandits and don't lose rep with the Legion.

Try to reset the game cache by holding LB+RB+X while the game loads from the console dashboard.
Anyway, consider that the legionaries you meet and talk with in Nipton are never hostile. Are you sure that your reputation with the Legion was neutral before that?