Well, the Die Hard movies proved you can just have the "wrong place, right time" senerio work.
A person doesn't need to have special powers to just walk into the wrong alley and get the wrong kind of attention. People in the real world all the time suddenly find themselves witnessing murders, saving people from murderers/thieves/rapists, jumping on top of people who are having seizures and have fallen on subway tracks, etc. Why can't an ordinary person walk in on a vampire or werewolf attack and actually manage to save the victim? Therefore attracting unwanted supernatural attention the same way someone who's witnessed a mafia crime attracts their unwanted attention.
Could you imagine what a supernatural witness protection would be like?
Die Hard movies. You can't use Die hard movies to prove your point. I love them, but let's face it- models of reality, they are not. They're an excuse to blow things up in more creative ways, and use a main character as a continuing thread in the story, to tie all those fireballs together.
An
ordinary person
doesn't save the victim. Ordinary people most often ARE victims; so it stands to reason that if they save someone, they have a quality that is extraordinary to begin with.
Most stories have vamps and werewolves being pretty fierce, frightening and strong, so unless the person is one of those gifted with preternatural abilities, it's not often going to happen that they can take them on and save a victim from the big bad. Part of the inherent scariness about monsters like that is that it knocks humans back down a rung on the food chain, and the potential hunter turns into the potential prey.
So an ordinary person most likely isn't going to be a hero. The hero is going to be someone with a lot of luck. Lots of bravado and smarts, but definitely lots of luck.