Others have written about the vampires described by J.K.Rowling within her wonderful world of Harry Potter, of Quidditch and Hogwarts and Diagon Alley. Yes there are indeed vampires in "Harry Potter" and one even makes an appearance in the books. But consider also how vampires in other fantasy worlds have wandered into the hugely successful novel/film series--by accident or in the imagination of fans.
Actors are an obvious place to start. Robert Pattinson was Cedric Diggory before he was Edward Cullen, of course, but going in the other direction Jaime Campbell Bower was Caius the Volturi before he became Gellart Grindelwald, one of the greatest dark wizards of all time (second only to Voldemort). As it happens, Bower is now engaged to Bonnie Wright (Ginny Weasely). Likewise Bill Nighy played Viktor in all three "Underworld" movies before finally getting to play Rufus Scrimgeor, Minister of Magic, in the upcoming "Harry Potter and the Deathly Hallows Part 1". Then of course there is Gary Oldman. Anyone not realizing he played a very prominent role in an important vampire film has clearly wandered onto the wrong website!
In one sense, however, matching up cast members to previous (or future) vampire films is just a version of Six-Degrees-of-Kevin-Bacon.
More interesting is how the whole subject of vampires become entwined with Harry Potter in the minds of fans. There is of course the speculation by more than one fan over the past decade that Severus Snape was himself a vampire. Such perhaps is the power of wardrobe. In fact YouTube sports more than one faux trailer for "Dracula" using Potter footage, nearly always with Snape as the Count and Hermione the object of his bloodlust. Dumbledore usually ends up being Van Helsing.
We can go further. All you have to do is run a few searches on Google or YouTube. Include the words "fan fiction" in your search. Pretty soon a list of such stories will appear, often in the form of crossovers. Again, Hermione is usually the object of the vampire's affections--be it Lestat, Louis, Edward Cullen, Alice Cullen, Damon Salvatore, Dracula from "Van Helsing," the title character in "Young Dracula," Dracula from Hammer films (i.e. Christopher Lee), etc. On the other hand, you can also find videos and stories with other pairings -- like Lucius Malfoy and Erica from "Underworld", for example, or portraying Alice Cullen as Harry's long-lost sister. Along the way you will also find a fair number of opening credits to fictional shows like "Hermione and Vampire Slayer."
So why the blend between the two?
On the one hand, both pretty much fall into subgenre of urban fantasy -- magical elements set side-by-side with the contemporary 'real' world. In that respect "Harry Potter" fits in right along side "Forever Knight" and "Buffy" as well as "Twilight" and even "True Blood" or "The Vampire Diaries." A certain amount of crossover in audiences is bound to occur. When that happens, some crossover fanfiction happens. Simple.
Dive a little deeper, though. Rowling created in her novels a bit of dream fulfillment. The orphan, penniless boy no one cares about learns he is wealthy, the famous child of beloved heroes, and the heir to great powers as well! With that comes the opportunity, duty and eventual necessity of avenging his parents' death. In the process he saves his new-found friends, finding true love along the way. A powerful brew, to be sure, but missing something. The vampire is (among other things) a different kind of dream fulfillment -- in this case, the not-quite-nightmare. The attraction is of the vampire as a darkness that can be understood, befriended, even loved -- that can understand you, befriend you, even love you. Rather than a hiding place for monsters, darkness becomes a secret hiding place. Dangerous maybe, but also welcoming. The nearest elements in "Harry Potter" that approaches this are the characters Snape, Draco and to some extent Thomas Riddle. No coincidence that so many fanfics center around redemptive romances with one of the three! Strip away the wizarding tropes and replace them with the stuff of vampire stories. Isn't what you end up with somewhat similar to many versions of "Dracula" or other vampire romances for the last decade or more?
Both Myers and Rowling appeared almost out of nowhere to write books that touched a nerve. Each said they were inspired by something intuitive. Myers wrote "Twilight" based on a dream. Rowling claimed to have 'seen' Harry with her mind's eye while aboard a train. Their books have sold millions and millions, many to the same readers. Whether both are female and devout Christians is coincidence or some subtly important ingredient are probably unanswerable. More clear seems to be that the nerves they touched with their works are rather close together -- in the same neighborhood as many other writers, including those intrigued by other vampire romances.