Vampires want to be human. They were cursed, infected, duped, whatever, but they now find their existence horrible and unbearable.
The only way a vampire can become human is by drinking human blood. After a feeding, their withered, pallid, rock-hard flesh becomes plump and pink and warm. They can feel, smell, and enjoy being human again, for about a week; then they slide back into their tortured but vicious nightmare forms.
In their inhuman forms, they are all but invulnerable and savagely deadly. Only in their human form are they easy to kill, as easy as killing any other human, but it is while they are in these forms that they most desire to continue to live. Only during the week of their blood-filled euphoria do they experience heavenly respite from their hellish eternal existence.
My idea here was to take the overused idea that vampires were smug and superior to humans (faster, stronger, longer lived, etc.) and turn that around so that all they want to be is just like us. We have what they lack, and they will do anything to get it, even though it is just a temporary fix to stave off their eternal unrest.
What do you think?