So, I have to talk to cpt. Curtis to do the I put a spell on you quest for the Legion but he attacks me on site, even with NCR armor. Is this quest impossible now?

If you have any bad NCR rep, I believe he still attacks you on sight, and I DONT believe he is fooled by NCR faction armor. What is your rep if I may ask?

Vilified.

That's why. Curtis still counts as NCR in faction logs, so if you have the bomb, you COULD just go to the monorail and run, but I don't know much about the Legion side of 'I put a spell on you'.

I don't have a bomb. Vulpes just sends me to Curtis for a mission.

To my knowledge there's no way to complete the quest without meeting Curtis.

He is not fooled by faction armour (even though Hsu is >_>) and will attack a villified on sight, even though he's meant to be Legion.

You can try getting your rep up with the NCR by doing things like retrieving the corpse of the kid, giving them NCR dogtags (not sure if that gives +rep actually...) and killing the fiend leaders in the quest for them (I know you can talk to the guy who gives the quest while disguised).

Other than that I know of no way around it unless you're on PC and thus have console commands.