Just curious.

According to the UESP article on vampires (lore), if they bite a victim and keep said victim alive after feeding, said victim is infected with the disease.

http://www.uesp.net/wiki/Lore:Vampire

However in the castle, it seems the vampires feed all the time on human "cattle" while keeping them alive. They sure don't seem to become vampires.

And of course the player can feed without infecting.

But perhaps these examples can be explained as not canon, since certain game mechanics are not canon according to lore.