The Parasite Blade also consumes magic, perhaps draining the supernatural force from their attacks.

The latter is your characters starting a conflict where there wasn't one. Inwas just trying to screw with Vash. The Servants were the ones starting a miniature crusade. Apparently I either have to fit the whims of other players or be attacked in-character for it?

This was going to happen sooner or later. Honestly better that they just hear it from the man rather than catch him in the act of feeding.