In a series that has had a serious rollercoaster in quality every season, Vigilantes is a chapter that is a low point in a completely different way than the main series.
The idea behind the show is a good one. A chance to have a hero plot like the main series, but lower stakes in small city. But instead it gets distracted constantly. The main plot was supposed to be the instant villains popping up and finding who's behind the distribution of the drug trigger, but it quickly shoved that to the side. It focused much more on the other characters and the little subplots they were involved with for an episode. Another issue carried over from the main show where the side characters get way too much time in the plot.
This is exacerbated by the fact that the main character basically does nothing the entire season. They throw in minor developments to how his powers could develop, but after teasing it a couple episodes it swings to the next distraction. He's constantly just being dragged around to help out various characters, but never actually seems to make an impact, or even defeat a villain in a way that feels earned. Even in the final major fight of the season he's not present, or even aware that it was going on, instead being involved with another side plot. Even Midoriya was somewhat involved with each season's major confrontations. Kouichi is basically invisible.
And perhaps most annoying of all is the frequent dropping of characters from the main series into the plot out of nowhere. It's as if they thought you would forget it was a MHA spinoff unless they added one of the teachers into an episode fighting a villain. And an origin story for why a particular character looks the way they do is hilariously added as if you need an explanation for a minor detail.
The show ends up feeling like a waste of time and neither separates it from MHA in a way that's interesting or supplements the main series in a satisfying way. It just ends up falling flat and doesn't give enough interesting directions the plot can go that can justify a person watching the second season.
