"A man had two sons . . .” was a common way to begin a parable, especially one comparing good and bad sons. Matthew uses it to contrast one son, who promises to work in the vineyard but never shows up, with another, who at first adamantly refuses to go to the vineyard but later repents and goes (21:28-32). Which one did the will of his father, asks Jesus? Not the one who talked a good game, but the one who actually followed through with obedient actions.