Klitschko replaces original opponent Canadian Kirk Johnson, who pulled out because of injury.
The Ukrainian had been due to appear on the card against American Cedric Boswell but offered to fill in for Johnson.
A fight between Lewis, the WBC heavyweight champion, and Klitschko had been tentatively scheduled for the autumn.
Klitschko had been the WBC's choice to fight Lewis but the contest had been continually put back due to rumours of a rematch between Lewis and Mike Tyson.
With both Lewis and Klitschko and the WBC itself all threatening legal action over the lengthy negotiations, the issue has finally been resolved albeit more by accident than intention.
For Lewis' part, he stands to lose up to $8million from the fight given the limited promotional time in place for the upcoming bout.
Lewis himself has poured disdain on the match-up, stating: "It's good to be back in the ring and feeling very fit and ready to go. The show must go on, for the sake of my fans out there, even though I am taking a pay cut by fighting Vitali now.
"Vitali Klitschko, as impressive as his record is, has never seen a boxer like me. He'll wish he had never asked for this challenge. I cannot see this going the distance."
The World Boxing Council (WBC) has finally got its way after Lennox Lewis agreed to fight Vitali Klitschko on June 21 in Los Angeles.
