I don't think Minutemen are vigilantes, but to answer you question, some of them are U.S. citizens - the rest are legal immigrants and illegal immigrants.
Isn't a vigilante someone like Charles Bronson who takes the law into his own hands when the US code is not enforced, and what is the effective difference between the Minutemen and the Neighborhood Watch groups which every law enforcement organization endorces? Seems to me the President should thank the minutemen for doing for the
Border Patrol what faith-based organizations do for welfare service. They are involved in civic activities, well within the white area of the law.