Vidic sat out most of the start of last season and was just getting his form and fitness back when disaster struck in Basel last December.
The 31-year-old suffered a cruciate knee ligament injury which ended his season and he managed only a handful of games this term before he was injured yet again.
Ferguson said: "In the last 18 months he has only had 10 games or something. It is not many games he has played.
"At the start of last season he got injured in the first game against West Brom and was out for six weeks.
"Now he has no pain in the knee - and its a different knee to the one he did his cruciate on.
"Last time he made the point himself - we went to examine it and he had to have the cartilage out."
United's defence has been a major headache for Ferguson this season.
They have conceded nine goals more than second-placed Manchester City and kept just four cleansheets in 24 league and cup games this season.
With a hectic Christmas ahead, Vidic will be expected to stiffen things up and Fergie smiled: "He's under pressure isn't he?
"But he'll make a difference, absolutely. He gives us the options.
"And to have senior players like that, it's helpful."