1. In bowl, whisk corn syrup, vinegar, the 3 tablespoons water, fish sauce, garlic powder, one quarter of the scallions, 1/4 teaspoon pepper flakes.
2. Place beef in plastic food-storage bag. Add 1/3 cup fish sauce mixture; seal. Refrigerate up to 30 minutes. Place remaining sauce in small bowl.
3. In saucepan, add remaining 1/8 teaspoon pepper flakes to 1-3/4 cups water. Bring to boil; remove from heat. Add rice. Stir; cover; let sit 5 minutes.
4. Meanwhile, separate leaves from head of lettuce; discard thick ribs. Rinse; pat dry. Rinse mint; pat dry. Chop some mint leaves to make 2 tablespoons and set aside. Trim tough stems from remaining mint leaves.
5. Pile lettuce, mint leaves, sprouts on platter. Transfer beef to platter; top with marinade in bag. Stir sweet-and-sour sauce, remaining scallions and mint into rice. Serve reserved fish sauce mixture for dipping wraps. Makes 4 servings.