I know one guy that can possibly bother wlad.. that's tua. considering his shaky chin anything is possible if you land that left hook right square on his chin. then.. K0.
They fight the best. It is just that right now the "best" is relative. They beat them all too. The era they are boxing in isn't their fault.
Tua can punch, but that´s it. He is to small and his footwork isn´t good enough. Jumping in throwing the hook won´t do it against either Klitschko unless he is really really lucky. They are too experienced for that and they have the skills (Wlad´s footwork; Vitali´s leaning) to avoid that punch.