You obviously haven't provided persuasive arguments against vigilante justice being a positive thing, in some cases
I wouldn't make the argument that vigilantism isn't valuable in some (usually hypothetical) cases. In cases like these, though, it isn't worth it. Taking the law into your own hands doesn't come with the necessity to grant the criminals the rights that they and every other person deserve, which is inarguably an issue. The legal and justice systems are designed in such a way that everyone is usually treated fairly. I say usually to allow for exceptions like police brutality and other similar offenses.
Who says criminals have the same rights we do?
The constitution. I'm aware that certain rights are "taken away" when a person is incarcerated, but that is only after they've been read their rights, been given a fair trial, proven guilty and sentenced.
