Ironically, in the past several years I've seen a deluge of vamp YA books that are simply a retelling after retelling of Twilight (Evermore by Alyson Noel springs to mind). I do agree that Meyer is more of a product of the adult market than the source of it. Twilight filled the gap in the YA market that 15 years of adult vampire fiction, the adult paranormal romance boom, and a growing YA trend had helped create.