Watching the first few episodes of Buffy this past weekend it is clear that Buffy is the template for Twilight. But Twilight is the anti-Buffy; a Mormon version of the Slayer which is purely English while Mormons can bes seen as a dissident sect of English. In season one number four a bus is taken over by religious folk refered to as the brethern; a name I believe used for Mormons in Australia.
Giles is also said to have a mentor named Carlisle - Edwards's father - back in the British Museum.
Noteworthy that Buffy's animus is called Angel; in book four of the Twilight series Edward and co. are referred to as angels and archangels. In both of these series some of the vampires can be seen as good or striving to be good - angels or Shining Ones - while in previous vampire movies the undead express a "silence of God" or empty crypt where the Christ once awakened from.
Worth noting as well that the Hellmouth mentioned in Buffy opens at the end of Pisces - 1996/7 while Twilight comes out on the other side - the beginning of the Age of Aquarius - 2006 or 7.
NOteworthy as well that Buffy is the active agent althought Angel kills Darla just as Edward kills Victoria allowing for the manifestation of the new goddess. Again, the Mormon model in Twilight where Bella simply finds her place with Edward.
This whole thing conditions a generation in a possitive way as both Buffy and Twilight bring the positive values of the white goddess and the beginning of the goddess cycle and bring to an end the age of Kali; the death mother. the generation which awakens and forms time, will manifest these positive values in the new century.