Becoming a vampire differs greatly on the world. In Buffy, the soul and the body are physically separated, and the body is joined with something of a demon. In the Hollows ones, vampires and their souls are separated as well upon death, and all undead vampires eventually get sunsick from missing their souls. Those ones are unbalanced, yes. But some vampires keep their souls, keep who they are, and it is their nature that has changed, not their souls.