Vidic will miss this weekend's showdown against Chelsea at Wembley and the following game with Birmingham because of a calf injury.
Wes Brown is also sidelined this weekend with a thigh problem but is expected back for the opening league match while Gary Neville is unavailable because of a groin strain.
Ferguson said: "Neither Wes or Nemanja have long-term injuries. I would expect Wes to be fit for the game a week on Sunday. As for Vidic he pulled his calf in the warm-up in Munich (against Bayern Munich) and is two weeks away.
"Gary is doing a little bit of training and coming on fine but I do not think he will make Sunday."
With goalkeeper Edwin van der Sar out for up to ten weeks with broken fingers, Ferguson has to choose between Ben Foster and Tomasz Kuszczak against Chelsea.
Both played a half in the game against Valencia in midweek and Ferguson added: "We have a selection dilemma as to who plays in goal on Sunday. It is not an easy one for me."
Ferguson also said he would be treating the match against Chelsea as a serious fixture and not as a test of his squad's fitness as he has done in previous years.
Following the summer departures of Carlos Tevez and Cristiano Ronaldo, Ferguson wants to prove to other elite clubs that United can retain their champion status without the two goalscorers.
"In the past I've treated the Community Shield as a barometer for fitness and giving players some minutes on the clock. But this year I'm not looking at it like that," the Scot told Sky Sports.
"All the players are at a good level fitness-wise and everyone should be able to give a good account of themselves. And when you play Chelsea, pride comes into it and the players want to win."