How do you unlock the Vampire Killer's full power? By defeating the Whip's Memory, which is a recreation of the last Belmont who used the Vampire Killer—in this case, Richter Belmont. He is often stated to be the strongest Belmont in the clan, only rivaled by Julius "Killed Dracula for Real" Belmont, yet Jonathan fights him alone. So when he finally gets to use the Vampire Killer properly, he's shown himself to be as badass as the Belmonts. Even better? Said boss is weak to the Dark element, and one weapon with this element is the Cream Pie. So you get to beat Richter with pies.
Also what the shade of Richter says when you beat him and unlock vampire killers full power.