Combine 2 tablespoons rice vinegar and honey in small bowl. Place beef Flank Steak and vinegar mixture in food-safe plastic bag; turn steak to coat. Close bag securely and marinate in refrigerator 6 hours or as long as overnight, turning occasionally.
Combine water, broth, onion, fish sauce, remaining 2 tablespoons vinegar, garlic, ginger and star anise in stockpot; bring to a boil. Reduce heat; simmer 30 minutes. Meanwhile, remove steak from marinade; discard marinade. Place steak on grid over medium, ash-covered coals. Grill, covered, 11 to 16 minutes (over medium heat on preheated gas grill, covered, 16 to 21 minutes) for medium rare (145°F) to medium doneness (160°F), turning once. Remove steak; keep warm.
Strain broth mixture and discard solids; return broth mixture to stockpot. Keep warm over low heat. Prepare noodles according to package directions using soaking or stir-fry method. Add noodles and any beef juices from steak to broth.
Divide soup among 6 large bowls; serve with sprouts, basil, cilantro and onions, as desired. Carve steak across the grain into thin slices and add to soup. Serve soup with chili sauce, Sriracha , hoisin and lime, if desired.
Nutrition information per serving: 586 Calories; 72 kcal Calories from fat; 8 g Total Fat; 3 g Saturated Fat; 3 g Monounsaturated Fat; 66 mg Cholesterol; 1483 mg Sodium; 93 g Total Carbohydrate; 12.5 g Dietary Fiber; 34 g Protein; 3.1 mg Iron; 7.7 mg Niacin; 0.7 mg Vitamin B6; 1.4 mcg Vitamin B12; 4.7 mg Zinc; 29.2 mcg Selenium; 103 mg Choline;