Vigilantism is a fairly modern concept that was developed based off of the ideas of Law and Justice we have had since about the 19th century.
All through history one can find what are deemed examples of Vigilantism though many is from us applying our own philosophical concepts of law and justice to ancient and older texts that were written with some vastly different philosophical views and applications.
This is especially a concept that has really taken off after the introduction of Super Heroes, and their popularity in present day. Many super heroes have undergone some transformations since their introductions in the late 1930s where they went from fighting fantastical ideologies of a culture's perceived threats in the world. They were often seen as some great benevolent force and was used to inspire people. Though soon they were starting to be translated into a kind of vigilante especially as some writes became discontented with current events going on in the world.
The modern concept of a vigilante only exists in the setting of Sirian. This is due to many of Sirian's locations having a similar structure of Law and Justice philosophies that mirror our modern day concepts, which in turn allows for the vigilante concept as we know it to be present.
However Vaxia is a more historical inspired fantasy game. While we don't base anything off an exact historical basis, Vaxia is set in a time that is similar to the Age of Discovery. While there is the start of some of the concepts that were eventually become our modern day law and justice philosophies, it is important to remember that it is not the same.
The concept of vigilantes as we know it today, does not exist in Vaxia as citizens of the various cities are highly encouraged to participate in the law keeping. This helps ensures that the citizens are aware of the laws of their city, it creates a further bond among the citizens which helps promote the idea of community, and it gave citizens a hands on way to see their system was working.
Mercenary groups that form with the idea of helping deal with crime is a common event and even encouraged often working with a local guard group if the city has one, or some equivalent that handles the more administration end of dealing with law breakers. Some towns and cities may even have it mandatory that anyone that can stop a crime is required to step in when they see one in progress, and should one fail to do so they could be arrested as a law breaker themselves.
One may say they are a vigilante, but the people will think they are just a mercenary/adventurer that is especially vigil in helping maintain the social contract (laws) of their community.
In essence all player characters are functionally vigilantes - it's roughly what an 'adventurer' is - so the notion of a caped crusader doesn't stand out in Vaxia.
It is important to remember that none of our settings are a 'Super Hero' genre game. Player characters are typically heroes and are "super" compared to most average citizens in either setting to begin with.
So if you are looking to play a more comic bookish gimmicky type of Super Hero/Vigilante our current Settings can not support that kind of concept. Some variations on the idea however may be able to work with some discussion and adjustment to fit within the settings.