Vidic suffered concussion during the Premier League meeting between the two sides at the end of April, an injury which kept him sidelined until the Red Devils' final game of the season at Wigan.
Despite forming a formidable partnership with England international Rio Ferdinand in a defence which conceded just 22 goals in the league all season, Kalou remains convinced that United's back four remains far from solid.
"We still believe that there is some weakness in their defence and that we can exploit that and score some goals," said the Ivory Coast international.
"They are like everyone, Ferdinand is a great defender, Vidic also. "But Vidic has not played a lot because he has been injured, so that could be a weak point for them and it is up to us to play well and score a goal."