Hands-on time: 30 min.; Total time: 8 hr., 50 min., including sauce.
1. Place steaks in a large zip-top plastic freezer bag. Whisk together fish sauce and next 7 ingredients. Stir in red onion, and pour mixture over steaks in freezer bag. Seal and chill 8 to 24 hours, turning once.
2. Preheat grill to 350° to 400° (medium-high) heat. Remove steaks from marinade, discarding marinade.
3. Grill steaks 7 to 8 minutes on each side or to desired degree of doneness, turning every 3 to 5 minutes. Cover loosely with aluminum foil, and let stand 10 minutes.
4. Cut steaks diagonally across the grain into thin strips, and serve in warm flour tortillas with Vietnamese Dipping Sauce and desired toppings.