most of the guys he's faced had trouble getting near enough to catch Vit clean on the chin, far less KO him, but as Sanders and Lewis proved, Vitali is not indestructable and a well placed punch could stun him
Prime Lennox, Prime Foreman, Prime Tyson, all would have a shot at it, in a fantasy match-up scenario.
It's tough to imagine, he takes a punch very well. Many fighters have a chance to KO him, it's just very very unlikely. Vitali has a combination of awkwardness and iron chin that you don't see often. Not only is he very hard to hit, if you do hit him he has a strong chin to back him up. Pretty special for a long reigning HW champ to not be KD'd ever.
Vitali is so good at Sensing when somebody is about to throw, he rarely get caught with Shots he doesnt' see. But i think one or two more Uppercuts like those from Lewis which snapped his head back would finish him.
Tyson, Bowe, Ruddock, Sanders, Lewis, Foreman, Tua, Shavers, Lyle, Cooney, Liston, Bert Cooper, Morrison, Oliver McCall etc.
Could anyone knock out Vitali? Not with his new "thumb your opponent then call for the ring doctor" technique. It's unbeatable.
Lennox and Sanders hurt him, so it's at least within the realms of possibility that they could also knock him out. Foreman, Tyson, Bruno, Ruddock, Witherspoon, prime Briggs, Gerry Cooney, Rahman if he landed flush, **** throw Oleg in the mix too. Haye probably, at this current time. I think shorter fighters like Tua and Morrison whose dominant weapons are their left hooks would have a harder time getting to him. Tyson's a slight exception here since his overhand right was so good. Of course, having the ability to KO someone and actually doing it are two different things. It's to Vitali's credit that he's never made a habit of having his chin seriously tested outside the Lennox and Sanders fights. But he isn't indestructible, and it's silly to even think that.