Caveat: The problem with analyses is that the explanation so often seems to miss the point. When something that is ultimately non-serious is taken very, very seriously, it runs the risk of falling a bit flat. What one loves gains a bit of, uh, blahness, as the final seasons of Buffy proved. (Uh, yes, I'm showing my bias there.) It's all right to get all maudlin and death oriented over vampires, but joie de vivre usually wins out in the end. Annette Curtis Klause's Silver Kiss is a much more substantial book, for example, than Charlaine Harris'Dead to the World (which is basically blood, guts, sex and more sex) but Dead to the World gets a bit closer to the mark in terms of catching the essence of vampire attractiveness.
In Harris'Dead to the World, the unreality of vampires is imposed onto the reality of a Southern town in a Sunnydale-like future. Robin McKinley's Sunshine attempts this same juxtaposition, in far better prose than Harris'. Klaus' later book Blood and Chocolate about werewolves succeeds for precisely this mixture of day-to-day living with super-natural living (vampires and werewolves aren't so much beyond nature as they are the embodiment of nature: very dead, very animal).
In other words, it isn't vampires, per se, that attract us, it's vampires and us. Dracula would be a cardboard monster if Stoker had used a vampire-expert as his narrator. Instead, he used a bunch of ordinary Victorians. It is their diaries, their excursions into Dracula's world that makes the book a classic. (For a hilarious version of Dracula from his point of view check out Saberhagen's The Dracula Tape.)
In Buffy, the vampires are more than cardboard monsters (bad-guys-who-get-killed) as their representatives Spike and Angel prove. Even more than Dracula, they provide a world, a society, which can be explored. The final season of Angel was redeemed from utter plot confusion by the continuing dialog between Spike and Angel. Being a vampire became, especially for Angel, not just a matter of personal redemption but a matter of social redemption, a construction of norms--this is what it means to be what you are.
The attraction between Buffy and Spike rests on a similar dialog and begins in the episode where Buffy requests Spike's advice re: how did the other slayers die? He tells her (and is subsequently hurt, emotionally, for his pains) by drawing on his experience. He invites Buffy into his world. His world turns out to be a strangely ordered one, in which Spike continually reaches out for the companionship he became a vampire to obtain but can never quite achieve.
In other words, the vampires of Whedon's universe are not merely lone creatures, searching, like lone gun men, for prey or revenge. They are explained by a theology, no matter how problematic. They belong to a system, no matter how flawed. They want companionship, not matter how incongruously that sits with their natures. Like Star Trek's aliens, they are a mirror to us ordinary humans but better than Star Trek's aliens, they are us, only dead. The mirror becomes exploration, not just reflecton.
And boy, doesn't that sound pompous!