Actually Wlad was never KOed but TKOed. Surely he doesn't have Vitali's chin but it's not like his chin is all glass. He can recover very quickly.
He learned to defend so well so even if some one manage to land big it will be very difficult to put him down. It would take someone with a massive power + powerful enough to come in and finish him - someone like Foreman or Lennox but I really doubt they will be able to land clean on Wlad because when Wlad is in the defence mode it is almost impossible to land clear on him...