Well, I'm at the Fort now, and I will randomly be attacked by the legion, Even though I bear the Mark Of Caesar, wearing Legion armor, and just walking towards my objective. I've tried wearing different things, and it didn't do anything either. I'd love to be able to get this achievement, but they randomly attack and I fail the quests.

Any way to resolve this? Other than loading my save about 5 levels back?

Theres a glitch with the Faction Armour, try switching between legion and Ncr armours then none and approach the Fort

That might be it? I don't know I've tried clotheless but they just shoot me. Oh well. I'll just reset the save from a few levels back. Thanks for the tip though, Papafern!