Vigilantism, by definition, has no accountability. It's an individual, who could be mistaken, doling out their own interpretation of justice. There are always exceptions to the rule. Sure, posing as a child could illicit attention from child predators, but it could also attract someone who is concerned about the welfare of the (fake) child. I can imagine someone, abused as a child, wanting to reach out and help someone they think is in danger of falling victim to the same. It's not a huge stretch of the imagination.

However, our legal system is woefully inadequate in addressing the amount of predators out there, precisely because it hinges on evidence of an act that often has none, while going unreported for long periods of time.

I'd say, that if someone is attempting to meet up with a child for the purpose of engaging in sexual contact, and you are alerted to that, you get to beat the shit out of them, but I'd draw the line at recording and posting, just in case you were wrong. Guy gets beaten up, learns a lesson, but if he was innocent, all he did was get his ass whooped.