" If reality changes, then everything changes, past and present, except for these little residuals like the clip."
Why would a "changed reality" affect everything - the original books, the VHS video covers etc., but somehow leave an odd YouTube clip untouched?
For what it's worth, I looked at those clips. Letterman speaks quickly and to my mind could easily be saying "the". It only sounds like "a" if that's what you expect/want to hear.
As for the author - leave aside the fact that there are copies of the original book from 1976 physically in existence - she also says "the". In fact in the same clip, she goes on to say "I always thought the vampire was the interesting one" It's a story about one particular vampire. That's why it's called what it is and always has been.
Seriously Mark. Unless this is all part of an elaborate and extended jape on your part, it's time to stop now and get back to the puns.