Vidic joined United three years after Ronaldo arrived from Sporting CP.
He recalled on Rio Presents: "If you remember when Cristiano arrived, he was obviously a player similar to Berbatov, who seemed to enjoy playing beautiful football, with his skills and his dribbling. His priority was not to score goals.
"The last two years of his career at United... he had totally changed. He was ready to go to Madrid and I certainly saw that he would do well."
Yet the former Inter defender also confessed: "I didn't expect him to do so well for so many years, he always had a mentality where he wanted to be the best footballer in the world. And he didn't hide it. He wasn't even afraid to say it in the locker room."
Ronaldo is now playing in Saudi Arabia with Al Nassr.