Ronaldo has scored 22 goals in all competitions this season and is already favourite to hold on to his PFA player of the year crown.
"We are very strong with him [in our side]. Last season was a great season for him, he scored so many goals. He was under big pressure to improve again this season and, up to now, he has played great in the games and in training," Vidic told MUTV.
"Every game you need to improve at this club. Everybody has sides to their game that they can improve and he has tried to fix his in training."