The 32-year-old, whose current contract is set to expire at the end of the season, has struggled with form and fitness this season - making just 13 Premier League appearances.
Vidic's agent, Silvano Martina, does not expect his client to pen fresh terms at Old Trafford and insists the Serb has already attracted interest from a number of other clubs.
"At the moment, I would rule out an extension with Manchester United," he told Radio Crc. "Vidic, given his contract situation, has several teams looking at him. Vidic is a very good player and will have no trouble finding a new club. For the time being we are remaining calm."
When questioned on speculation surrounding a switch to Napoli, Martina denied any concrete interest on behalf of the Serie A side.
"At the moment there has been no contact," he stated. "But thanks to the management of [President Aurelio] De Laurentiis and [sporting director Riccardo] Bigon, they are one of the best clubs in Europe."