nah, what's a little poison among friends right!?
I think a neurotoxic would affect both parties.
Destroys nerves. I think regardless of whether the body lives I think.
A vampire would cuickly regerate it, but that still slows him down a little. A human would be outright paralised. If they dont go into shok right away. Pretty lethal shit :)

Only downside it's expensive. No problem for you.