Mr. Echo40–I congratulate you. Just when I thought I had read it all on this forum, I read this opinion. It is certainly not one we see everyday on here. Although I do worry it will lead to "what is the best caliber for a Vampire, Werewolf, or other Unholy creature?" (I did not know you have met my ex-wife and her family).
Thank you, and I'll gladly preemptively settle any potential of such an argument taking place with the following statement...
There is no best bullet for reliably stopping an unholy creature such as a Vampire or Werewolf because both are characteristically impervious to any form of permanent damage by mortal weaponry due to their aforementioned regenerative capabilities.
I could make a joke here about .45 Colt/ACP being the best due to it being "The Lord's Caliber" but in the interest of maintaining a serious tone for a serious — albeit outlandish — topic, I will remind everyone that such a sentiment is merely a joke. I can assure you as a devout Catholic that Scripture contains no references whatsoever to God having a favorite or otherwise approved caliber.
Based on my experiences being so unfortunate as to have been born into a family of monsters, I can say to you assuredly that the only thing I've ever witnessed that is capable of repelling an unholy creature are all things of God. Crucifixes, Scripture, and objects of devotion blessed by an ordained minister will work, provided that they are backed by Faith. Fortunately, encountering such creatures tends to make a believer out of anyone.
That being said, for the purpose of identification, please note that descriptions/depictions of such fell creatures have been largely dramatized in novels/motion pictures. To the naked eye and untempered mind, Vampires and Werewolves are practically indistinguishable from Human Beings. In addition, as far as I've seen, drinking blood as well as devouring flesh is a symbolic representation of their actual method of feeding.
Keep in mind, these are supernatural entities, ergo they feed not on flesh and blood, but rather the essence of humanity, their emotions, particularly negative ones. They feed by means of evoking powerful negative emotions from their prey, ergo any bodily harm they cause by doing something so monstrous as drinking blood or taking a bite out of someone is merely a means to that end.
So yeah, you can shoot them with just about anything and the most it will do is temporarily immobilize them, and even that much is going to require such an absurd amount of wounding potential that it frankly isn't worthy of consideration.
You carry firearms for mortal enemies, not undead. That being said, I don't think it could hurt to outfit your firearm with engravings, grips, striker plates, or magazine baseplates inscribed with holy verses or imagery as an improvized secondary repellent.