After you confront Benny at The Tops, Vulpes Inculta will find you and give you the Mark of Caesar, which will remove all of your Legion infamy. If you get vilified after this, however, there is no way to get back on their good side. You can still wear a Legion disguise to get into their hangouts without getting shot though.only if its before you have yet to meet Caesar. or else the best reputation you can get is wild child
You'll reset to neutral when you first arrive at the strip. Otherwise you can wear a legion disguise and try to do quests at Cottonwood Cove, but the guys who hand them out are also the guys who can see through disguises, so it'll be very hard to get them to talk to you.