I'm guessing something like the 3rd Fury fight, with Wilder getting stopped late. Vitali, comfortably ahead on points, wins by KO11, with Wilder having his moments, landing the more eye catching shots, but not enough of them. Klitschko's chin was good enough to withstand the few isolated bombs he's hit with here.
I think Wilder could last long enough to lose a clean UD but suffer heavy punishment in the process, Vitali is a boxer who wins by crushing his opponents but I think Wilder is tough enough to last until the end...
Vitali - Probably by stoppage but, as to when he closes the show, it's hard to tell. Wilder seemed to have a knack for hanging in there a while, despite suffering heavy KD's.
Agreed that Wilder losing a UD is not out of the question. I don't think Vitali was as explosive as his brother. Guys like Adamek TKO10, Sosnowski KO10, Arreola TKO10, Gomez TKO9, Donald TKO10, Bean TKO11, Puritty TKO11, all managed to make it to the later rounds w/Vitali (worth mentioning that Vitali was the only fighter to stop Bean & Donald). Don't really see any reason to believe prime Wilder can't get there. I think Vitali wins it either way though. FWIW, I think Wlad had a better chance at stopping Wilder in the early to mid rounds, but was a lot more vulnerable to getting blasted out himself.