Buffy's hardly Sci Fi, though is it?
I mean, you'd have to start thinking about the vampire population of Sunnydale, the rate of slaying and how often they need to feed and so on. The numbers simply don't add up, ...
Now; I'm aware of a whole sub-verse of SF categorisations out there, but it's a subject I bored of very quickly. Whatever; if you have criteria for "Sci Fi" that include
The Numbers Must Add Up, I reckon you're going to struggle with Dr Who, Star Trek, and er ... 90% of the characters suggested so far!
given the vast quantity of Vampire-fiction taking over our screens/Kindles now, I propose that Buffy would be lumped in with them by most consumers, rather than as "SF" - but when it was
written, I bet it was discussed in most of the same magazines as Star Trek et al.