You just don't get it, do you?

Vampires have too many weaknesses. Ninja don't have weaknesses at all. Except maybe yielding to a higher authority of some sort.
Ninja are stealthy: they'll sneak up on a vamp and slit their throat, then gut them with their katana or ninja knife . . . or they'll just use razor wire to rip the vamp's head off.
Ninja are chemists: they'll concoct a "potion" that will easily rip a vamp apart from the inside, out.
Ninja are cautious: you say "sissy star-throwing pajama-wearing nightowls," I say "gigantic boomerang-launching flame-repellent armor-wearing super-fast blade masters with a tendancy to attack from long range."
Ninja are never alone: the shadows are a ninja's friends