Decide on a general direction for your plot. Vampire movies have split into a number of subgenres, including vampire villains and vampire hunters, vampire clans and corporations, heroic vampires and vampire love affairs. There are even vampire comedies. You can combine them all into your movie, but you need to focus on one for the main plot line.
Reduce your concept to a single sentence synopsis: "Lonely vampire aristocrat finally meets a human he loves," "vampires invade remote military base," "teens discover there teachers are vampires." A synopsis focuses your thinking during the screenwriting stage and becomes your pitch when its time to sell the script.
Determine your target audience. Developing a movie with a specific audience in mind will not only influence the directions you take with your screen play, but make your movie easier to sell. Young adult girls like romantic vampire heroes, you adult boys like heroes or villains as long as there is action and gore, older adults prefer fast pacing and suspense.
Plan your lead characters. You need a heroic lead who typically is a vampire at war with other vampires, a vampire hunter, or an innocent civilian who discovers the vampire threat. You should include a romantic interest who might be the human the vampire loves, the hero's soul mate who is threatened by the vampire, or the hero's estranged lover. Detail is important, write at least a page to develop each character.
Develop your supporting characters and setting. Good screenplays convey visual detail; setting and characters provides much of it. Supporting characters typically include the hero's rival, a colorful sidekick, skeptical family members and police, other evil vampires and victims who end up getting killed because they refuse to pay attention. Settings can be anything from trailer parks to remote outposts to theme parks so long as there are plenty of dark corners to conceal evil.
Outline your movie in three acts on note cards or a word processor. Throw in a plot twist to keep your audience from guessing the outcome (the vampires are victims, the vampire hunter the mass murderer; the hero's confidant or lover is a vampire). In act one the threat is discovered, in act two the hero's efforts to defeat the villains are thwarted and in the final act victory is assured.
Write two drafts your script, making changes to your outline as needed. Write in screenplay format, and keep the script to 90 to 100 pages (about 90 to 100 minutes of screen time). Read the first draft out loud before you revise it. Listen for errors in dialog and pacing and cut scenes that don't drive the plot.