Is there any reasonable way to add a script to an NPC Vampire without editing every vampire in the CS? I already thought of running getnextref and then adding a token object that has a script, but I don't want to kill the performance of the game by running it all the time.

One approach would be, each time the game is restarted, add an extra script effect to one of the ability spells common to all vampires.
If the script was extremely trivial I'd handle everything inside the effect script; otherwise I'd have the effect script check for the presence of a scripted token and add one if not found, letting the token script handle everything else.

That sounds perfect, thanks!