People saying Ferdinand is "miles" better are bonkers, he's a better defender from open play but from set pieces and when the ball is in the air Vidic is equally far ahead. Lets not forget just how many aerial balls Vidic wins, something Ferdinand at his best could never do. Team's cant play long ball against us because none of their strikers ever win the first ball to get the knockdown, part of our problem before we got Vidic was the opposition just used to be able to plonk the ball up field and their midfielders could fight for the knockdown in our half, when Vidic's playing this doesn't happen.
Ferdinand is the better defender but he isn't "miles" ahead. Just because getting your head to everything isn't glamorous doesn't mean it isn't vital, it requires good awareness and excellent positional sense (something people seem to be criticising him for here), as well as a lot of determination and bravery to be willing to put your head where it hurts regardless. Lets not forget Vidic isn't like Jaap Stam or Chris Samba, he's not a massive unit which makes his total dominance aerially it all the more impressive.
I'd say a defensive partnership of two Ferdinands wouldn't be much better (if at all) than two Vidic's and both would be considerably weaker than what we have now. Where Ferdinand put's distance between himself and Vidic is his organisational skills, but then it's easier to be an organiser when you're playing along side someone as good as Vidic 99% of the time rather than Smalling or Evans.