In my opinion, those who choose to call the Minutemen, Vigilantes, do so to demonize and discredit them-period.
I suspect you're right, since "vigilante" has a totally negative connotation, in spite of it's having sometimes been a pretty good idea. However, to me, the historical meaning of "Minutemen" (not to mention the Minuteman missile :-) is clearly suggestive of violence. I think that people engaged in standing around in the way and phoning the cops should have chosen a different term.