However, I do have a very well-defined idea of what a vampire is and is not.
I think that's the big reason that I didn't like Twilight when I read it, and why I have no desire to go see the movie (although I don't think it's in theatres any more). She took the vampire rules, and mostly threw them out the window. Sure, she tried to explain how the classic vampire myths are just myths and "this is how they really live," but it really didn't sit well with me.
I've started reading a new book, called Dhampir, and although it is not entirely true to the classic vampire definitions, it doesn't throw everything out. In the mythology of this book (and I suppose the rest of the series, though I'm only on the first book), vampires exist, and they follow the classic rules: can't be out in the sun, can only be killed by a stake or beheading (although there seems to be an exception for a magical weapon? I'm not far enough in to know yet!), don't like garlic, drink blood, etc. The difference is that they are one of the Noble Dead, which are made up of several types of undead things. There is a breed of half-vampires called dhampir, which are bred to hunt vampires. I like the idea of a special critter made to kill vampires, but I'm not sure I buy into the 'half-vampire' thing... again, not far enough into the story yet to know how those are created (aren't vampires sterile?).
I think it helps that I've played some D&D with vampires in the stories... D&D vampires are somewhat different than traditional vampires, but again, the main traits tend to be similar. There are different breeds of vampire, and each is susceptible to a different type of damage, but at least it's from the traditional sources.
In any case, I've discovered that I can enjoy non-traditional vampire stories as long as a) they incorporate most of the vampire traits and b) are well written!
I've also discovered that I have far too many opinions on vampires...