L.H., I don't think what you say about Buffy is true. I think she heard Lloyd's version of the song, thought it was about a vampire, and therefore said it was about a vampire. Certainly the vast majority of people who heard Buffy's version did not know the song in any other substantially different version; it was collected from Oral Tradition in North America, but those versions never went anywhere in the revival; meanwhile, all the British versions that have been recorded are based on Lloyd's, as is Buffy's. So I do think that most people who first heard Buffy claim it was a vampire simply believed her because she said so. They did not select from among other interpretations, because she said "this is about a vampire," and they took her at face value. Perhaps it's a bit strong to say they "fell for it," but it's equally strong to say that they chose to believe that interpretation because they liked it. The fact is, that's what they were told by someone who, probably erroneously, believed it to be so.   

Reggie, thanks for posting the lyrics. You're right, of course, that my musings above may have been irrelevant. Your friend may have based the song on some pre-existing tradition connecting vampirism and what were traditionally called "night riders," or he may not have. Either way, it's a good and spooky song!