vex - rather, “throw into consternation” [Gesenius].
make a breach - rather, “cleave it asunder.” Their scheme was to divide a large portion of the territory between themselves, and set up a vassal king of their own over the rest.
son of Tabeal - unknown; a Syrian-sounding name, perhaps favored by a party in Jerusalem (Isa 3:6, Isa 3:9, Isa 3:12).