Place steak cubes in plastic food-storage bag. Whisk soy sauce, oil, vinegar, sesame seeds, sugar, garlic and red-pepper flakes in a small bowl. Reserve half to use as dipping sauce. Add remaining marinade to plastic bag. Seal; turn bag to coat meat well. Refrigerate 4 to 6 hours.
Prepare outdoor grill with medium-hot coals, or heat gas grill to medium-hot.
Meanwhile, cut corn and zucchini into 1-inch pieces. Cut each scallion into 3 pieces, each piece about 1-1/2 inches long.
Cook corn in boiling water to cover for 3 minutes. Drain off water.
Thread meat cubes onto 6 metal skewers, alternating meat pieces with corn, zucchini and scallions. Brush once with marinade from plastic bag.
Grill kabobs, covered, turning occasionally, 12 minutes or until vegetables are cooked and instant-read meat thermometer inserted in meat registers 145°F for medium-rare. Serve with reserved marinade for dipping and hot cooked rice if desired.