Study the vampire romance genre. Read as many vampire romance novels as you can. Stephanie Meyer's "Twilight" series and Shannon Drake's books are good examples. Note what you like about each book and what you don't like; this helps you from falling into cliches and can inspire you to put your own twist on the genre.
Research vampires. Studying vampires in general will give you another perspective that you won't get from reading only vampire romance books. Bram Stroker's "Dracula" is a good place to start since it spelled out the original rules for vampires. Vampires can be sadistic and cruel with an unrelenting thirst for blood. They're not just good-looking, romantic creatures.
Flesh out your characters. In most vampire love stories, the protagonist is a female human who falls for a male vampire; instead, you could write a love story in which two vampires fall in love. Vampire love stories also must have a villain. The villain could be another vampire determined to keep the couple apart or a human vampire hunter. Your supporting characters could be other vampires and the protagonist's human friends and family. Write a detailed bio for each character, including a physical description, personality, likes and dislikes, motivation and anything else you feel is important.
Create a flow chart or outline to organize your story's time line, plot development, conflict and resolution. Each point in your outline should have a one- or two-sentence description about what happens at that point in time. This helps you keep your story on track.
Begin writing. Find a quiet place to write undisturbed and set daily or weekly goals. You don't need to write your vampire love story in sequence. If you feel inspired to write about your main character discovering that her love interest is a vampire before you've written the chapter where they meet, feel free to do so and write the prior chapters at another time. Your writing doesn't have to be perfect---you'll go back and edit later. End each chapter with suspense to keep your reader reading.
Edit your work. Read your story aloud to catch grammar mistakes and awkward sentence structure. Add sensual details to make your love story stronger. Invite a friend to read the story and ask for her thoughts. Or, seek out the help of a professional copy editor.