The Serbian limped out of the opening-day win at West Brom with a calf injury.
At the time, it was estimated Vidic would be out for six weeks.
However, that deadline is now approaching and the defender is no nearer a comeback.
"It's slow progress with Nemanja," revealed Ferguson.
"He's not done any training. He is still getting treatment on the calf.
"Calves can be troublesome. I remember when Bryan Robson had calf trouble it kept him out for three months.
"Hopefully it's not as long as that, but he's not making any great progress at the moment."