The Korean shop keepers were not practicing vigilantism. They were not roaming the streets or conducting rogue justice. They simply protected the own property from attack, from native-born Americans, not illegals, whose ancestry likely goes back further than yours or mine. That is perfectly legal under the law.
And much of what I have heard of the "vigilantism" along our southern border consists of private property owners whose property is continually trashed by uninvited migrants traveling through. If such a property owner needs help to protect their property is that not allowed? How does owning property along the border obligate one to tolerate trespassers?
If such migration is truly illegal, then nobody should criticize those who would use the necessary force to stop it. The people who are crossing at these places are no doubt crossing there because, at least to some extent, the law IS being enforced elsewhere along the border.