Which is how neighborhood watch should work.
These aren't vigilantes. They are people protecting their homes, families, and property.
Unlike the scenario you described above, these were two men who armed themselves and went out to confront Arbery without consulting or working with the police. They weren't trying to help law enforcement, they were trying to do their job for them. That's the very definition of a vigilante.
They were working with the police though (and there was a 911 call). In fact, the police had referred neighbors to the senior McMichael because of his background.
As said, the police are counting of this activity because they know they it takes time for them to respond to the commission of a crime.
Most of my neighbors are edgy about this matter and it wouldn't be too surprising to learn that someone was shot during this time. Lot's of retired folks live around here and they are nervous about their safety.
I am in Georgia BTW.