By exposing, I mean showing them up in a highly public subreddit/community/forum etc with solid proof (direct links showing them grooming someone they knew was underage, archived voice clips, webpages, screenshots etc) and getting that community to instantly turn on them, or in some cases getting them banned from the community because the mods saw the evidence and decided that they don’t feel safe having that person in their community.
Also, is vigilantism generally seen as a good or a bad thing?
So to start with the “also”… vigilantism is mob justice. It is always a bad thing because it denies people their due process.
It is very, very rare for vigilantes to get it right, or to have excellent target selection. The vast majority of vigilantes go after innocent people whose sole crime is to be different- because they’re bigots.
That said, assuming you didn’t otherwise break the law (so no breaking and entry, or hacking etc) it would most closely be investigative journalism. Sure it might get taken down on whatever platform, etc, but that doesn’t make it vigilantism