“A” is used in some translations, I don’t know why.
True, it is a story about a whole bunch of vampires. Why am I so picky about this… “a” just sounds wrong to me, y’know?
It ends with Louis as a vampire in isolation. Has he isolated himself to prevent himself from being manipulated by others? Or to prevent himself from hurting others? There’s plenty of reasons to isolate oneself.
Part of the point of “the” is probably that Louis actually is more special than he thinks. Like AR herself, he passively let a lot of things happen in his life, but did grow throughout the course of the novel, and by the end he’s very much gained his own autonomy, in ways he himself may not even fully realize. I think this is why AR never wrote a whole novel from his POV again, she had pretty much exorcised those demons.

[^X scan @thesaramonster]
It was also a great way to piss Lestat off, how dare Louis refer to himself as “THE” vampire. Lestat makes it very clear that he’s the one that should have been interviewed, giving himself “The” as the very first word in the title of his novel ;]