I would like to talk about the meaning of gaiden again. This is going to be relevant to this discussion, so please bear with me.
The following video is a talk IGA did in 2014 in which he talks about the development of SotN.
https://www.youtube.com/watch?v=gLyjAWYK2KgFrom 3:00 to 4:05 he talks about how Konami was divided into seperate studios. The SotN team was situated in Tokyo, but the "franchise owners" were the developers from the Kobe studio. They were the ones from developed the "proper" games in the series. Because the SotN team didn't view their game as a legitimate entry, they "figured they could do whatever they wanted as long as they treated it as a seperate series" (this is what is written on the slide). IGA and Yamane both refered to SotN as being a gaiden on several occasions (in English it's usually translated as "side project" or "side story"). For example,
here,
here, and
here. In the last link IGA helpfully clarifies that with "gaiden" he means "sub-series". So that ties into what he said in the above video. But what exactly does this mean? When I had the chance to ask IGA some questions a couple of years ago, he gave an answer that seems to clarify this. This is what he has to say about the various gaiden games in the series:
IGA: 黙示録、黙示録外伝、Circle of the Moon、Order of Shadows、THE ARCADEは、世界観を同じくした別の世界の物語となります。黙示録、黙示録外伝、Circle of the Moonは、製作者の意図として、そのように扱っております。 Translation: Regarding Castlevania 64, Legacy of Darkness, Circle of the Moon, Order of Shadows and the Arcade, they are stories from a separate world with the same world view. Castlevania 64, Legacy of Darkness and Circle of the Moon are treated in the same way as the makers intended.
IGA: 漆黒たる前奏曲は、ドラキュラの世界観をまったく無視して作られた作品だったため、私がプロデューサになった時に、正しい歴史からはずしました。あくまで同じ世界観をつかった別の世界の物語と考えてくだされば良いかと存じます。 Translation: Because Castlevania Legends was a work that completely ignored Castlevania's look, when I became producer, I removed it from the proper history. I think it would be best to think of it as a story from a different world that used the same look.
The translation is a bit different, but in both cases used the term: 世界観 ("sekaikan" or "world view"). This term is clarified in one of the books that I own: The Untold History of of Japanese Video Game Developers.
Sekaikan! There's been a discussion in English on the importance of that word. It has deep layers of meaning: the atmosphere, world lore, world view, the background behind things.
So what I gather from this is that the way IGA uses gaiden, he refers to a game that share the same sekaikan (the atmosphere, the look, possibily also the same lore) as other games from the Castlevania series, but which he treats as a "seperate world". Furthermore, he seems to imply that, in the 90s, the games made in Tokyo (Rondo of Blood, Vampire Killer and Symphony of the Night) were gaiden while the games from Kobe were the official entries.
This brings us back to the discussion at hand. The above situation that IGA described seems to have strong parallels with the Vampire Killer situation. The game was refered to as a gaiden and also rebranded to make it a seperate series. Once again, all the "in-game evidence" that supposedly proves the game wasn't gaiden doesn't matter because in the very definition of gaiden it says a game might have the same "seikaikan" as other games but is still treated as "not legitimate" i.e non-canon or a parallel universe.