Yes, calling them “vigilantes” with the modern negative connotation is stretching it.
"...but supporters say they're trying to protect America from "an invasion."
He didn't mean that was a stretch did he? A Balkanization is an invasion, the same as Kosovo-- its an invasion that exploits a nation's lack of barbarism.
Negative connotations? How about this, from an article I read a few minutes ago:
Many of the Minuteman volunteers were recruited over the Internet and some plan to be armed for protection. (ABC Action News)
I think that is supposed to have a negative connotation. Probably does, to the average left-wing idiot.