The thread topic had me scratching my head a bit, so I actually looked up the word "vigilante" on the presigious Google search engine, and first result was this:
a member of a self-appointed group of citizens who undertake law enforcement in their community without legal authority, typically because the legal agencies are thought to be inadequate.
This definition does not seem to automatically imply violent attacks against perps, but rather the establishment of a level of law enforcement at a level lower than, let's say, the municipal. The clause I find puzzling is "without legal authority", because it sort of opens up a rabbit hole. Someone once asked me maybe 15 years ago "what is law", and, realizing this was meant to be a deep question, I was obliged to answer with "I don't know." At the time I could not assess how to trace the final kernel at the heart of the word "law". I could do better now, but the fact remains that the chain of contingency in any establishment of law sort of needs to be understood in a discussion of when it can be circumvented. oldbrian just above wrote that "lawful authority comes from the people", but to me this seems like it's making an assumption. If law is an artifact, constructed by people, to achieve some end, then of course it comes from the people; and if it isn't, then it doesn't, and it comes from somewhere else. Law in the form of municipal, or other social authority, still stems from somewhere even if it differs from what we might call natural law, or divine law, or whatever else.
The New Testament, for example, makes a statement about a possible divergence between divine law and Roman law (give unto Caesar etc etc), but even in a context like that we have the interesting example that Roman law had baked into it that local peoples like the Judaeans should self-govern to the maximum extent possible within certain bounds. This type of concession was not to be nice, but among the probably various reasons would have been that it would be inefficient to try to micromanage a people like that against their will. So we see a level of governance, and at a lower level a "self-appointed group of citizens who undertake law enforcement in their community without legal authority". Now, it was sanctioned, so in that respect it was
with legal authority, but in the sense that they using their own local laws exclusively in determining guilt or innocence (the Jewish law) and not actually rendering the higher-level Roman law as part of their procedures, they were effectively operating outside of the legal authority (in the sense of the legal system) of the Romans. So was this vigilantism? Were the Jews in this context okey-dokey morally because the Romans said ok, but if the Romans had been against Jews using their own laws, would rabbis secretly applying those laws under the Romans' noses been morally in the wrong? In other words, did their moral right to apply laws according to the code of Moses derive from the Romans, or was it merely the ease of them doing so that came from the laws of the Romans?
I know the term vigilante usually has a Batman or Punisher connotation, of going out and beating people up, but perhaps that's only a subset of what we might call establishing a lower level of legal authority below another one and separate from that other one. How much the 'higher' one fights against that lower level would depend on the situation. The Romans appeared to be savvy enough that it's better to let the local law go with their blessing rather than fight it, so this wisdom (if you want to call it that) may be been strategically to their benefit, but I'm not that from this we can deduce a moral authority (or lack thereof) of the smaller legal group.
Going back for a moment to the broader question of "what is law", a materialist might certainly find themselves stuck in something of a bind defining law as anything other than the application of force against parties that violate the agreed upon rules of those with the greatest force. It's not so much a war of all against all, as an establishment of retribution that is well understood and consistent. If you do X then Y happens to you. From a strictly mechanical standpoint all laws only have one real clause: follow the guideline or you will be imprisoned, hurt, or killed; and since allowing oneself to be imprisoned is in a sense voluntary (since you can resist), the law when boiled down to its most fundamental component is "do this or die." From that standpoint I find it difficult to establish any moral authority intrinsic to this that makes a lower-level legal establishment (like a community tribunal, let's say) somehow wrong, per se. It might violate the "follow our rule or die" mandate, but it's hard to directly establish that refusing to follow a directly coercive directive makes you bad. It might BE bad, in the sense of highly disruptive, or bad FOR YOU, in the sense of dangerous, but it seems tough to establish a moral color to vigilantism if law is only a human artifact that only has value insofar as it is a rule with a penalty attached to it. The moral authority of it has to come from somewhere other than that for it to be intrinsically wrong to establish a different legal authority below or beside it. To say that "you're breaking the establishment rules by establishing your own law" is just a truism, not a moral statement. I suspect that most people who would think of it as an automatic moral statement also believe (at least tacitly) that the mere fact of a law being a law makes it good. This itself would have many premises baked into it that would need unpacking to even discuss it.
Sorry LR, is this kind of answer on-topic as far as your OP is concerned?