Jason, I don't anyone thought at the time it was offside, including me. But it does appear that when Evra headed the ball, Vidic was further forward than the ball and defenders and so the fact that it hit him and diverted into the goal makes it offside.
It was close and the referee and linesman can't really be blamed as the ref was at the wrong angle and the linesman couldn't see Vidic through Evra, and probably no-one thought it hit Vidic anyway.. But it shouldn't really have been given. Rather like the United winner at Stamford Bridge a couple of months ago.