The advantage of an armed citizenry is that it is capable of assembling itself quickly into a group of whatever size is needed to deal with present circumstances, and then disbanding such group when it is no longer needed. Permanent police forces are at any given time almost always going to be either understaffed or overstaffed (since instantaneous demands on the police will vary far more quickly than staffing levels). I would guess long-term vigilante groups would probably have the same problem; if there isn't anything for them to do, they'll "find" something.
Pretty much what happened throughout American history.
Extended-service vigilantes tended to provoke the formation of counter-vigilante groups, often called “Regulators,” with the conflict between the two groups sometimes escalating to something close to civil war.
Still, it’s difficult to see any other solution where the collapse or absence of government makes conditions intolerable for the average person.