I highly doubt Ennis set out to impose any sort of law about what "real" vampires are vulnerable to in Preacher. Mentioned it briefly before, but the best Cassidy story was were he met a more recently spawned vampire who conformed to all the gothic expectations, and declared him a "tosser".
I still like the Vlad The Impaler/heresy Dracula origin. The idea of a person become corrupted by turning their back on God. Yeah, it's Christocentric (is that a word) but it's a damn sight better than Emopires and could easily be applied to certain modern Bible bashing imperialists in comics to hilarious effect. (Please note, I have already copyrighted Vampire Dubya, hands off.)