So problem 1:

This is Mafia 33. I have 32 games to draw from for very concrete, very demonstrable examples of characters with a night kill that were neither a) a vampire nor b) a member of a faction.

It's not like "vigilante" or "serial killer" are even obscure roles or anything.

And problem 2:

If the Night Kill ability does not come from being part of a faction - and since her Watch faction ability wasn't revealed, it's almost certainly doesn't - then why should we be lynching vampires? Even if we were to accept your unfounded assumptions that every vampire has a Night Kill, how is that even relevant to anything? The town wins when the scum faction is eliminated, and the best evidence suggests that vampires are not the scum faction. They are currently the least suspect of all players here. And that fact is reason enough for any remaining vampires not to use any Night Kills they may or may not have because that would instantly lose them their best defence. What is it you wish to accomplish? I've never before seen a game with so much dedication to lynching a player because she is suspected of possibly being a vigilante. Hell, even when Isaac got the town to hunt a phantom Serial Killer instead of the mafia he wasn't this zealous about it, and I still immediately and correctly suspected him of being scum for it.

What is your objective in this game, beetnemesis? You've been chomping at the bit to get vampires lynched from Day 1, with less and less reason to do it. I'm very much entertaining the suspicion that you are the one who Night Killed Stephen because you simply hate vampires that much.

Would it make you feel better if the Watch investigated players to get a vampire registry? We can investigate players to discover their race or nationality, and I'm fine with tighter control over lethal actions. But really, I think even if they can, the vampires would have to be really stupid to try to night kill anybody right now.