Wanted the opinion of a person who has more knowledge of Vampires and vampiric lore. And you even said normally the removal of a Vampires heart means their death. (which I guess is sort of a play on the old cliche of putting a stake through their heart) I read up on a Kain and remember that he was a detached soul placed into a corpse of a vampire. So he's not a "normal" vampire. So that makes sense he can't be killed by normal vampiric flaws.
Indeed, the only way in this forum to vanquish Kain is if you destroyed him completly by turning him to dust or something. As long as you can turn his body into smaller molecules than what he can control like mist then he would be vanquished.
Although do not take me as a knowledge on Alucard himself, I know little to nothing of Alucard, ime better knowledge at Legacy of Kain vampires is all. Although I imagine draining all his blood would leave him pretty weak.