Put quite simply, Evil is that which works at odds to the Creators design.
There are certain behaviors and actions that have been shown over and over again to be evil or at the very least negative in the long run for the participants willing or not. But it's certainly just as evil to ignore such behaviors.
But that's the crux of the matter. According to Christianity, God is all knowing, God is all powerful. He knows the past present and future of everything, of everyone. When he created his most beautiful angel he knew that satan would fall from grace and become the father of lies. This says to me that satan was incorporated into the design. He had to have been or he couldn't exist. God is everywhere and in all things, that would include satan.
So in our theoretical world where vampires exist, they must therefore also be part of the design, because nothing can exist and not be part of the design. The problem isn't the design, but our own arrogance and presumption to know the design, without which knowledge, we cannot judge for ourselves.
Gives a deeper meaning to "Judge not lest ye be judged yourselves", and "Let he who is without sin cast the first stone."
Man's vanity caused the Crusades, something that they believed was God's will, and yet we now question and apologize for.
We slaughtered people thought to be witches, because the bible said "suffer not a witch to live among you", but that was wrong too, because the bible didn't say suffer not a witch live.
There are many more examples where man performed atrocities against his brethren, believing it to be what God wants.
Maybe in the end, we haven't got a clue as to what God's design is, or what he wants. Which would mean that our concept of evil is dependent merely on our point of view.
I'm a Christian, but I know that I don't know, and cannot know the design and heart of my God, because I am imperfect. We as a whole may want to curb our arrogance before we commit the next atrocity.