User blog comment:Crimson Marauder/Vampires wielding cursed weapons???/@comment-25942629-20160105003847

Crowley wields one just fine while waiting for Guren to show up. If I remember correctly, vampires cannot form a contract with a demon or use the demons like the humans do unless they are contracted prior to being turned into a vampire. This is what happens with Mahiru. Once someone translates Catastrophe volume 6, we will have more information about this.

As for the vampire weapons, I don't know jack about them, but I doubt the humans would appreciate being stabbed through the hand and going anemic all the time. The vampires heal and drink blood just fine, but the humans cannot.