Well vampires are hardly true imortals they still have physical bodies. If the body is destroyed and the ashes scattered the vampire is in essence dead. A demon prince however cant die. well they can be defeated in the matrial world and they go back to the realm of chaos. And maybee they can be killed and absorbed in realm of chaos but thats beyond mortals to achieve.
As for no souls? then where did their souls go? I remember reading that the vampires are in fact a merging of a predatory spirit and the vampies own soul (vampire coutns armybook 6th edition).
The part about having to bend knee is however a large issue but lets not forgett that tzeench made a deal with nefretta to give her the last piece of he puzzle for the elixir of life (6th edition armybook). Also the vampire wouldnt have to give them his own soul, a person cna make deals witht eh dark gods offerign up the souls of others, we see Morthi doing that in the sundering. She doesnt worship the chaos gods she makes pacts with demons as an equal.