I under stand that with using V.A.T.S it makes multiplayer Co-op difficult to add to the game, but you could just add a targeting system to multiplayer Co-op mode.
The other player or players that are with you would compensate for not having the time stratigize while using V.A.T.S. And as for the story its self (if thats an issue)
you could have the missions player sensitive, by this I mean instead of the quest giver saying "Fred, I need you to blow up Caesers Palace". It just changes to
"Fred and Steve" (bad name e.i.)
I loved Fallout 3 and I will definetly be playing Fallout New Vegas reguardless of Co-op options or not.
P.S. whens Elder Scrolls 5 coming out?