The Serbia international centre-back announced his decision to end his eight-year association with the Premier League champions earlier in February.
United's outgoing captain was reportedly disappointed with the lack of movement on a long-term contract at Old Trafford and subsequently announced he would play for a side outside of England.
Inter emerged as favourites for his signature, and his move to San Siro is seemingly nearing a conclusion.
"I can't saying anything other than the deal for Vidic is 90 per cent done," Thohir told journalists at Milan's Malpensa airport.
"When everything is sorted we will make an announcement."
Vidic has enjoyed a fruitful spell during his time at United, collecting five Premier League titles and three League Cups, as well as enjoying success in Europe when he lifted the UEFA Champions League in 2008.
Knee injuries have plagued Vidic in recent seasons, though he shaken them off to make 23 appearances for David Moyes' side this term.
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