All the Minutemen are doing is watching the border and calling the Border Patrol, not even making a citizens arrest. Not vigilantism.
Clayton Cramer, who knows a thing or two about American History, points out
Vigilantism in American history has often been the result of there being no effective criminal justice system (for example, in the California mining camps in the first year or so of the Gold Rush), or where the criminal justice system was so corrupt that it might as well have been absent—of which the San Francisco Vigilance Committee of 1856 is a good example. It formed after a county supervisor shot to death a newspaper publisher in front of many witnesses—and the District Attorney refused to prosecute. The federal government needs to make a serious effort to secure our borders, so that people like Haab [who had made a citizen's arrest, but was not part of the Minuteman Project] aren't tempted to take the law into their own hands.