The Nerazzurri confirmed on Wednesday morning that they had agreed a deal for the Serbian defender, who will move to the San Siro on a free transfer at the end of the season.
Vidic, who signed for United in 2006 from Spartak Moscow, has won almost every major honour in the game during his time at Old Trafford including the Champions League in 2008, five Premier League titles and three League Cups.
And speaking to Sky Sports News Radio, Berg believes Vidic will be a huge loss for United.
"He's still vital to the club so I don't think it's a football decision," he said. "He's been there for a long time now, so I think it's simply down to the player wanting to do something different.
"He's a fantastic leader and a top player - one of the best central defenders that they've had - and I'm sure he'd like to go out on a high note."
Berg believes the departure of Vidic at Old Trafford can draw comparisons to the time when Jaap Stam left United for Lazio.
"He could still play on for United. Like Stam, I believe he's leaving when there's more years left in him. But I don't think this is a decision made by the club, or [David Moyes] the manager; this is purely down to Vidic wanting to do something different."
The full interview can be heard here: