Vidic had been the subject of constant transfer speculation last season, with Real Madrid believed to be keen on the central defender. United moved to end the uncertainty by handing him an extended deal.
Sir Alex said: ``In the modern day game it is not easy renewing players contracts when they do well, particularly those from other countries. Over the last year or so there has been speculation about Nemanja time and time again, sometimes by the press, sometimes fuelled by other sources.
"But we never had a strong feeling he really wanted to leave. There is always concern if they do well they want better contracts. At the end of the day we have managed to achieve the task of getting him to agree a four-year contract.
"We are delighted, he is fantastic centre-half, a great competitor with a marvellous attitude to winning games. It strengthens our hand.
Sir Alex also revealed that he expects Rio Ferdinand, who was injured during a training session with England before the World Cup kicked off, to be available in early September: "Rio should be six weeks, I think. It was a ligament injury after all.''
Gary Neville returned to training on Friday after a calf injury while Antonio Valencia will be back at the club's Carrington training complex on Monday. On Michael Owen, Ferguson said: "He is behind Gary at the moment and I am not sure he will make the start of the season.''