The Lords of Shadow bestiary states that vampirism has to be accepted, which is why only intelligent creatures can become vampires. How exactly is Trevor turned against his will?
Perhaps being a Humanoid AbominationPhysical God lets Dracula bend the rules. The bestiary for this game states that he's trying to create a "purer" breed of vampire, which is why those vampire knights retain their humanoid appearances, so there is a precedent to Dracula doing things that go against the established logic of this world.
If Alucard doesn't need to breathe, why does he gasp for air when he comes up out of water?