[denethorthefirst]

It was always my impression that Thuringwethil was simply a Maia of Melkor, obviously less powerful than the first (Ungoliant, Gothmog, Sauron) or second tier (the Balrogs), maybe on par with the Boldogs. The "Bat" was simply the form she chose, nothing more, nothing less. Tolkien may have been inspired by vampire-folklore, even Sauron has a brief stint as a vampire in the First Age, but Tolkien only borrowed some characteristics and outward appearances. Neither Thuringwethil nor Sauron were "Vampires" as we commonly understand them and as they appear in modern horror fiction. I also think that such "vampires" wouldn't fit in Tolkiens world, it just feels completely alien to Middle-Earth, but thats just my feeling.