Jamieson, Fausset and Brown on Isaiah 18:5

5. For--rather, "But."

perfect--perfected. When the enemy's plans are on the verge of completion.

sour grape ... flower--rather, "when the flower shall become the ripening grape" [Maurer].

sprigs--the shoots with the grapes on them. God will not only disconcert their present plans, but prevent them forming any future ones. Horsley takes the "harvest" and vintage here as referring to purifying judgments which cause the excision of the ungodly from the earth, and the placing of the faithful in a state of peace on the earth: not the last judgment (Joh 15:2; Re 14:15-20).