You've gotten farther than I have with this - I did take a look at the coding and at first glance I don't see anything in the tuning or in the Python layer for either invading vampires or on servos that would prevent servos from being able to zap vampires, unless the specific vampire you're trying to zap is pregnant.
From what I remember, vampire invasions were broken before Discovery University launched and were left that way by design (as per the bug report forums), so I doubt this functionality was ever actually tested by the dev team.