It hit me the other day that vampires could drink milk because it is actually a specialized form of blood. It has all of the nutrients needed, and is pretty well accepted today. I also realized that in germanic lore, the nosferatu is more scary because he is undead than the fact that he drinks milk. I am fairly certain that vampires in the Germanic lands didn't even notice a change since they drank so much milk anyway.
The rest of the world is lactose intolerant, so the vampires had to drink something else to feel safe. The only choice they had was blood. Go ahead, think about it for a while, your mind will be blown.