^ One could argue that "death" can be delayed but never stopped, which is why the PCs are able to beat him back but not destroy him. Frankly we could really use more info on those stones from LoI. Sigh.
Edit: Oh, yeah, there's also the indication that Death doesn't have a complete hold on anything, what with Elizabetha being reincarnated into Lisa and Dracula into Soma, though one could argue that Death is actually responsible for that, too. Would make sense.
As far as any of us know the Vampirekiller is the ultimate bane of evil, like the Master Sword is in Zelda.
This is a decent comparison, considering how the VK was tied to the Belmont line during the ritual that forged it, and the Master Sword went through something similar. Leon could not harm Walter with anything less than the complete VK, much like in WW when the weakened MS couldn't damage Ganondorf. Leon said he would 'kill the night'; I think it's safe to say that all creatures of the night are vulnerable to the VK, save perhaps incorporeal critters like ghosts and such. Death would be no exception to that.
Edit: There's also a possibility that Death is vulnerable to it because of the 'living' soul within the VK. VK has been demonstrated to have some strange sentience.