You seem to be under the impression that there's only 2 agruments here when in reality there is at least 3.

Your interpretation seems to be that Either Vigilante justice is never OK, or vigilante justice is always OK.

No one here is arguing that vigilante justice is always OK.

The argument here is between vigilante justice is always OK, and vigilante justice is sometimes OK.

The examples of slave revolts and lynchings of black people both fall into the camp of sometimes. Slave revolts are always morally good, while racist lynchings of black people are always not morally good. Both were illegal at the same time, but at no point is it argued that both are always morally good.