The myth of vampires not having souls, or losing their human souls once they turn to the darkness, is false to an extent. It’s only technically true for First Generation Vampires, and their pure bred offspring, who have a demonic souls that resides in Hell. Otherwise a vampire that was once human has a soul. Certainly they are human in the majority of parallel Universes since being a vampire is quite rare. Human souls are unified spirits comprised of their near infinite lives lived out in all the various parallel realities, and dimensions. All mathematical probabilities are played out within a certain parameter determined by various higher dimensional Gods, and even ourselves. Therefore we’ve all been vampires in some other Universe.
Despite vampires not losing their human soul they are in essence cut off from it. Their human DNA becomes demonic based, and their normal Astral Energy body is replaced with a dark demonic energy body. A dark energy linked to the demon who originated their particular line of vampires. This dark energy more or less hijacks the connection to their higher dimensional soul, and instead creates a new connection to Hell that is more direct than the connection to their human soul. This, and the inbred urges in their demonic DNA result in many vampires being evil. Thankfully there are those who reject this demonic connection, and are sub-consciously in tune with their human soul because no matter what free will remains. These good vampires often fight against the forces of evil turning the tables on the continued effort to recruit new demonic minions.