"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.
Whether or not Luke had some version of Matthew in front of him as he wrote, he uses the same opening formula, precisely to subvert the expectations encouraged by its literary genre. Luke, master storyteller of the New Testament, knows exactly what he’s doing here. First Jesus’ opponents articulate the same binary logic suggested by the opening formula; then Jesus tells a parable beginning with that formula that proceeds to blow binary thinking right out of the water.