Castlevania and Vampire Killer were in development around the same time with their release dates only a month apart - it's not a big deal if people shuffle the order around a bit when the gap is so minimal to begin with.
I enjoy Vampire Killer, as you would any curiosity of its ilk. It's a game where you either revel in its inherent obscurity or become infuriated by it, perhaps ending up with a happy mixture of both. Visually the game shows up a lot more subdued than its sibling - the palette is full of earthy browns and depressing greys. It manages to differentiate successfully from the source, but also lacks any sort of memorable flair in stark contrast with its vibrant counterpart. The stage architecture is also a lot more compartmentalized and boxy, playing with the rules of the platform by necessity, which is unfortunate considering its aspirations for being a sort of a mystery puzzler. Combatwise, I don't think it could be more awkward, another quirk to call its own.
It's a game that's held back in a lot of ways by uncooked, half-realized ideas and execution, but it's so novel and unique that it deserves anyone's time of day, at least once.