Its mostly sting operations, pretending to be kids online and when they get groomed they ambush the predator with all the evidence, upload the footage to the net and give everything to the law.
So while in some cases they do get a beating, mostly these people don't use violence and have surprisingly good standards of evidence. The police welcome most of them and they do deliver convictions. If done right of course. Those that don't rely on the police for a resolution are less common but do exist.
I can think of one instance where an innocent man was murdered because a loony thought he was a molester, I'm sure there have been other cases of violence which is why overall the best solution is the real authorities to step up.
The groups that basically stalk and trap these types but leave the resolution to the police, they seem more reasonable and just seem to be doing what the police don't have the manpower for. But more serious vigilantes do exist and that is less reassuring.