I think hes one of those guys that throws consistantly solid shots. Not a one punch KO boxer, but one who discourages his opponents when he lands, which allows him to set up more shots, which KO in accumulation. Although at HW a solid shot is nothing to sneeze at.
Vitali has decent power. He ko's his opponents with accumulation. There have been loads of heavyweights with better power. Many just don't have the combination of size and skill to lay hands on their opponent like Vitali does. Again, his power is decent but not great. He needs loads of clean shots to ko good fighters. Huge punchers like Tyson and Tua can do it with only a few punches.
Actually the more I think about it, Haye is in with a at least some chance. Sure his chin isn't granite, but its not like Vitali is Mike Tyson. Seeing as Haye is willing to throw bombs from the opening bell and hits pretty hard himself, his punchers chance is better than most. Vitali has a great chin, but I can't see many people taking a flush Haye right without being slightly stunned (at least). But I do admit it is a bit of a stretch.
Vitali has very good power, probably better than we've generally seen. He is also a very measured, calm and deliberate fighter that doesn't go for "one punch" knockouts -- he doesn't need to. He has tremendous ring generalship, patience and intellect. He has and uses the tools to carefully rip people apart, round by round, stopping them once they are tired / hurt and demoralized. Vitali is a puncher that causes massive damage by accumulation as part of his strategy. Haye might commit more to punches, hence he is often unbalanced and vulnerable, but I doubt that he can "hit harder" against a heavy bag.