Now that's gotta hurt, vampire or not. I am reminded of the famous scene in Terminator 2, where the T1000 gets a clean hit from Arnold's grenade launcher.
It's also spinning. It's easier to penetrate something like that. It's kinda like a drill, except that it's going to explode.
Well...not quite? The spin just increases its stability in flight, kinda like an american football when thrown flies better with a perfect spiral than when its wobbling all over.