I think this is entirely the problem...
No, it isn't, I just updated Civ V just fine, and other steam games play alright, only New Vegas seems to be effected.
I have put in a support ticket, but I don't expect a reply. I wonder if we will ever hear a peep out of Steam or Bethesda about what the heck is going on and why a new game disc owner, can't play their single player game after installing on their PC.
I just purchased the game today (on disc) so I had a new game for my week of vacation, didn't realize it was a steam game, looks like another game has been thrown into the "steam"ing pile.
Thanks for throwing your customers under the steam bus, Bethesda