Are you a vigilante when you call the police because your home is being burgled?  The Minutemen operate on PRIVATE property with the permission of the property owner.  When a tresspasser is found on that property owner's land, they hold the lawbreaker and call the police.  They are not enforcing the law, but calling the law enforcement officials.  If the Minutemen were operating on public property, you would perhaps be correct.  But that is not what they are doing.