The Serbia centre-back played for his country this week in their Euro 2012 qualifying defeat to Slovenia, having been absent from club duty since August due to a calf injury, but a start at Anfield on Saturday is far from guaranteed.
"The big question mark is whether Vidic is ready for the game tomorrow," Ferguson told a news conference.
"He's certainly had his first game for Serbia on Tuesday, played a full game, so it's certainly a consideration because he's such a good competitor.
"He came in yesterday and felt OK. But the Liverpool game is such an intense, draining game that you want everybody to be 100 percent so it maybe more down to how he feels himself really."
Even if Vidic does not play, Ferguson has plenty of other options and was quick to point out that Jonny Evans had been "fantastic" in the heart of defence.
Midfielder Tom Cleverley, who damaged ligaments in his ankle a month ago, and defender Rafael remain sidelined although the Brazilian has now returned to full training after missing the start of the league season with a dislocated shoulder.
Ferguson declined to be drawn on Thursday's news that his striker Wayne Rooney would miss the group stage of England's Euro 2012 campaign after being handed a three-match ban by UEFA for his sending-off against Montenegro last week.
"I'm here to talk about the Liverpool game," he said.
GAME OF SEASON
Saturday's encounter is even more keenly anticipated than usual as the bitter rivalry between the clubs has been cranked up a notch since United won their 19th English league title last season to overtake Liverpool's record.
Liverpool have won the last three meetings between the clubs at Anfield, including a 3-1 victory last season, and Ferguson said the electric atmosphere at the ground played a big part.
"I think the fans play a part, there is no doubt about that," he said. "In my time we've had nine players sent off there which is unusual for Manchester United so that is all down to the atmosphere that can be created there."
The historic importance of the two clubs always made their meetings emotive matches, Ferguson said.
"Although things may change in the next few years with Manchester City, whether the derby game can be equal to Manchester United-Liverpool game, because I've always considered it to be the game of the season in English football and at the moment I think it remains that," he added.
United are top on goal difference from Manchester City after a blistering start to the season but have looked less convincing in recent league games against Stoke City and Norwich City and the Champions League home draw against FC Basel.
The unbeaten Manchester teams, who meet at Old Trafford next weekend, have 19 points.
