A sub-genre appears to be emerging from the vampire revival, as more and more vampire writers go for series instead of just the one-off, with very deliberately open-ended plots, and given the extraordinary post-Anne-Rice, post-Buffy commercial book-to-screen successes of Stephenie Meyer's sparkly teenage vampires and Charlaine Harris's Sookie Stackhouse, why wouldn't you.
David Wellington did this (again) with his most recent novel 23 Hours, in which the fiendish 300-year-old vampire Justinia Malvern escapes in the end to drink blood another day and the righteous Laura Caxton likewise survives and escapes custody, presumably to chase Justinia down through the next volume and so on ad infinitum.
And now we have Jasper Kent's Thirteen Years Later, sequel to Twelve (they do love their numbers, these vampirists) and second in a projected 'quintet' (Kent is a composer and musician as well as a novelist). It's as though the 'vampire novel' form were declaring itself, like its subject, conditionally immortal; I guess stories of the undead just naturally lend themselves to open-endedness. I haven't read the Kent books, but they look like classy generic hybrids: historical horror fiction, up the literary end of 'genre'.
My friend and former colleague Prof Ken Gelder published a book about vampires in 1994. Given the current crop of vamp lit in all its diverse flowerings, he'd be crazy not to be thinking about a sequel himself.