Okay, I'll admit it. I'm an old-fashioned sort of girl. I like my monsters to be, well, monsters. Vampires should be creatures of the night. They don't go out in the light, at least not without getting a really serious case of sunburn, and they most certainly don't go to high school. Werewolves are ruled by the cycle of the Moon and shifting from human to wolf or back again isn't quick and painless.
So, faced with the split in current literature -- don't hit me for using the "L" word. I mean it in it's most general, least insulting way. Truly I do. -- how do you go about setting the stage for your vampires or weres?
The first thing you need to know as the writer is how your vampire or were (or shifter) came to be.
There are any number of legends out there that can be the basis of their metamorphosis. A person can become a werewolf (or other type of were) through magic, a pact with the devil or a demon, a curse, or through infection. American Indians have legends about skinwalkers who hang the pelt of the animal they want to become on their belts and through various means become the animal when putting on the pelt. There are legends about the seventh son of the seventh son or those conceived under the new moon or born on a full moon Friday the 13th becoming weres. Then you have genetic mutation, both natural and of the mad scientist type. There are others, all of which make good fodder for the backstory of a novel.
Vampires can be turned by another vampire -- we're all probably familiar with this one. Legend also has it that if a child is conceived during a holy period or if that child is the illegitimate offspring of illegitimate parents, he will become a vampire. As with weres, there is also the mutation theory as well as any number of others out there to play with.
Another issue you have to consider as a writer is what happens to your character after he's been "changed" or "turned". How does this affect his humanity? A were or shifter is human when in human form. At least initially. But what happens when he's in his animal form? Does he maintain the ability to think and reason? Or does he become fully animal? Does his humanity decrease the more he shifts and does there come a time when, if he stays in his animal form too long, he can no longer shift back to his human form?
Versions of the same questions can be asked about the vampire. Does death, and possibly the loss of his soul, remove his humanity? How does this affect him and his interactions with the humans he is going to come into contact with? More importantly, if he doesn't have a soul, can he relate to those non-vamps in your story in a way that seems even vaguely human (yes, I know this is a religious and moral issue. I happen to think it is one you have to deal with). Another question to ask is what being basically immortal will do to your vampire. Is the human mind capable of maintaining its sanity over the centuries, especially if your vampire has any sort of emotional ties to humans over those long years?
With regard to your characters changing into the appropriate "creature of the night" (no, this is not a cue for the song of that title from Rocky Horror), how does the change occur? I just saw an ad for the latest Twilight movie where the emo were takes a flying leap and changes from human to wolf in the blink of an eye. Of course, the shiny vampire was there too and, oh, it's daylight. Sorry, back to the topic and away from my pet peeves.
Does your were shift only during the full moon? What are the mechanics of the shift? Is it slow and painful? Do you take into account mass to mass ratio? In other words, does it violate the rules of your world for a 5' tall, slender young woman to shift into a mouse -- or into a mastodon? Does your vampire change into a bat? If he does, is it only one bat and a normal sized one at that or a number of bats? Sally, in Terry Pratchett's THUD!, turns into 130 bats, some of whom occasionally fly off, making it a real pain to re-materialize as a "human". Another interesting twist on the common vampire mythos is that only the male vampires reappear fully clothed. For some reason, the female vampires don't. Which can make for some embarrassing situations.
Other issues to deal with are deciding if your world knows about the existence of the weres or vampires. If they do, how did they learn about the existence of the "monsters" and how are they adapting to that knowledge. The early Anita Blake books by Laurel K. Hamilton are good about showing how the world is slowly admitting to the knowledge of weres and vamps, if not exactly accepting them. If the world doesn't know about them, how are they trying to keep their presence a secret? This is especially tricky in this day and age of forensic science and video cameras on every corner.
What other issues do you see in writing about werewolves and vampires? What do you think about the current trend of making them "human" with extra powers ala Twilight and its ilk?