I am villified by the legion and I whenever I show up to the fort I am fine until I run into a Praetorian Guard or a Mongrel and they attack me for no reason. This is especially annoying since the water pump station is full of praetorian guards. And as soon they attack the rest of the camp does and I can't take them all on.

what do?

PS who am I supposed to talk to to get the Platinum Chip back?

i assume youre wearing a disguise. praetorian guards and mongrel dogs can see through the disguise. plus, if you are vilified, it may be impossible to fix unless you killed some NCR people.

does killing NCR gain Legion rep? Or does it just make me lose NCR rep?

It just makes you lose NCR rep. If you're using a disguise, you should be able to kill the guys in the shed by themselves without alerting everyone else, so long as you keep your disguise. Maybe you have to unequip then requip. But I know I was able to handle them that way.