So, definitely not a weapon you'd see frequently (or even uncommonly) used, but I think we can call the Vampire Killer at least "Plausible" in ye olde Mythbusters parlance, and it would probably be a truly DEVASTATING weapon in the hands of someone who was well trained with it. The sheer skill, strength, dexterity, and technique required to use this thing effectively would probably contribute to the Belmont's legend in-universe all on its own, and the rarity of such a weapon would make a Belmont really stand apart from the crowd as soon as they pulled it out.