Taking fiction aside, if there were a real-life non-lethal vigilante, how would the criminals they catch legally go to prison? The vigilante broke the law to gain evidence, so all the evidence the vigilante obtained would be thrown out, and every criminal would walk. As messed up as it is, The Punisher and Dexter make sense, because the only way a vigilante can really be successful is if he just straight up kills the criminals.
Usually in these stories, Batman or whoever leaves behind enough evidence to support a successful prosecution, along with the tied-up bad guy.
That’s actually an interesting situation. The fourth and fifth amendments put restrictions on the government, not private vigilantes. So if the cops just happen to find evidence in plain view, there won’t be a direct constitutional reason to suppress it.
Now if the local prosecutor has a pattern or practice of deliberately turning a blind eye to the unlicensed private investigators that routinely supply them with illegally obtained information, there’s probably a claim there. But it’s a lot more complicated to make that case than a straight-up 4th amendment case.