The Enclave got their asses kicked on both coasts of the US, I don't see them ever coming back.
EDI has a voice recording that talks about an Enclave base in Chicago(Setting in Fallout: Tactics). I don't know if it's really big, or how many men they have, but it's not East or West Coast.
Also, I think New Vegas is way better than Fallout 3, if you can somehow ignore the quest bugs, animation glitches, and other incessant problems. I can't tell you how man times I reloaded my Hunting Shotgun and had to wait like 15 seconds AFTER it's done before being able to do anything, like shooting, crouching, going to inventory, talking to Boone or EDI, searching containers and bodies, etc.. I'm also pissed off about my "Rifle-type" weapons going above and below my view in first-person. In short, there needs to be a patch to fix all the tiny instances of bullstuff that happen all the time.
