Uhm.... Just commit crimes against the faction they war with, so attack NCR, ect ect. And sometime soon, you'll be recognized for a quest, that erases all previouse explotation you've done to ceaser's legion. If you haven't got that then, just attack the NCR, and opposing factions to the legion. And you will get a quest, asking you to join - and or, stating if you do what ceaser says in that quest. Then all previcouse attacks on the legion, will be deminished... And they'll be friendly to you again. That' or just cheat it out. Lol, you probably don't have a clue what im talking about.. but... whatever. Lol