Well their soul reavers and two soulreavers together often means something strange will happen. I would say Kain is the superior swordsman because we don't really know much of vorador or his combat skill, he has little feats. Sure he beat malek but how good was he really? again, we don't know other than he was a sarafan champion, a militant order so potentially just an especially skilled human. After that hes secluded away.
Kain brings physical superiorities natural to him outside of magic but also, the fact he fought for years with the reaver blade against humanity, eventually conquering them, he could have been little more than a general in this but at least he was not in seclusion, doing little.
Then theres the fact he did quite well for himself even as a human, he fought off a large number of assassins all at once for a time, eventually they killed him but theres few who could under normal circumstances take on so many at survive for any period.
The main advantage to kain is the list of melee skils in Defiance though, he can swing his blade like a blur back and forwards, uses his "float" (are you considering it magic? its natural to a vamp) in-combat and has a variety of other moves suggesting Kain has the ability to learn a lot of martial skill.
Don't forget also, Kain drunk the blood of countless sarafan in BO 2, their strength became is, and potentially their knowledge of combat.
Well in the intro movie of BO he jjust sort of knocks him on the head, later when Kain summons him you don't watch because of course playing as Kain your charging forwards to deal with the circle.
I can only imagine he used a combination of magic such as his teleportation and his sword to dispatch him.
Also I am not sure if its explained yet how he appeared in BO 2. That said, don't forget Kain can come back from pretty much anything, maybe Vorador has a similar way of doing so.