not sure when but id bet vit stops him within the distance. vitali's handspeed makes him look like Ali compared to that slow oaf.
I'm not sure if anyone can KO the big guy but Vitali, Wladimir, Haye & maybe Peter have the best chance.
Vitali is definitely active enough and tenacious enough to stop Valuev. He would have no respect for his power or skills either, because Nikolai doesn't have much of either attribute. VK by TKO inside the distance.
I would say Vitali actually has a shot at KO'ing Valuev merely on the implication that Valuev has never faced a serious puncher, his power is not enough to earn Vitali's respect, his fight with McCline indicated that he was ill-prepared to deal with someone who is on relatively equal footing with him, Valuev's speed is not enough to keep Vitali honest. Valuev's chin is solid but he never gets to match up against someone who isn't punching UP at him or able to control the distance or pacing.
I'd actually love to see that, like a KobeIsGod said, Vitali would look so fast. Not only that but he would be the most powerful figher that Valuev would have faced and I can just see him having no respect for him and his lack of speed and power. Mid to late TKO or KO. It's gotta happen. If there is one boxer right now that you do NOT want to land at will on your face it has to be Vitali - and that is what he would be doing to Valuev. (disclaimer: I wouldn't want any boxer to land at will on my face)