Pretty much this same thing happened to me with Dune Scorpion. I would play the Dune Scorpion, Quint it, then SoW, but no Neurotoxin. I was confused and annoyed at first, but then I tested the same thing with Forest Scorpion and again, no poison on spell damage. I didn't think about testing it with Vampire but it seems to be along the same lines. My guess is these are "On-hit" abilities and Spell Damage is not treated as an attack, so these abilities aren't triggered. I'm not sure though, it's only speculation.
