Sketch out your basic plot. Vampire hero plots typically include a vampire who by choice or coercion is forced to stop a bigger threat to humanity. Replace the formula elements with specific details (for example, a vampire driven by guilt over his condition who discovers his sire intends to turn humans into cattle).
Create your characters. Vampire heroes and heroines follow two models: the genuine vampire hero like Jack Fleming who starts out trying to solve his own murder, or a morally ambiguous vampire like Anne Rice's Lestat who does everything he needs to survive. Typical supporting characters include a human love interest and an obstacle character (vampire slayer who can't see the hero's good or a reporter trying to ferret out the vampire's true identity).
Build multiple levels of conflict. Heroic vampires usually square off not only against the villain but also against the love interest conflicted about her romance with a vampire and against the human authorities unwilling to accept the existence of a supernatural threat.
Outline your novel to distill your ideas and work through potential problems. Novels unfold in three stages: the setup, escalation of conflict and climax. Vampire hero novels usually unfold in the following outline: the vampire becomes aware of the danger, his attempts to stop the villain backfire and he finally finds the villain's fatal weakness.
Start typing. Don't worry about getting all the details right, just get the words on paper. Work in twists and surprises (the hero's human son is the villain, the villain has connections to the mayor). End each chapter on a cliffhanger to keep the reader reading, or make sure she picks up the book first thing in the morning.
Write a one-line pitch and a one-page synopsis to sell your book to an agent. Don't forget to plan the sequel or series.