The reason the main series Vampire Killer turns from leather to metal is because it's made via alchemy. It's the same weapon, same whip, just magic properties. It changes form in Lament, for starters. Richter has it in metal form in Rondo and in Symphony his sprite has metal but his art has leather (they just took his Rondo sprite).
Also, the SCV4 Simon's Vampire Killer is made of metal, if memory serves.