How you slice a piece of meat is key to how tender it is, especially with tougher cuts. Thin slices cut against the grain (across rather than parallel to the muscle fibers) are more tender and easier to chew.
Finely shred zest from 2 of the limes; juice to get 1/4 cup. In a small bowl whisk together 1 Tbsp. of the fish sauce, 1 Tbsp. of the sugar, lime zest, oil, and garlic. Place steak in a resealable plastic bag; pour mixture over steak. Seal bag, turning to coat. Marinate in refrigerator 15 minutes or up to 24 hours. Drain; discard marinade. Grill steak on rack of a covered grill directly over medium heat 15 to 18 minutes or until desired doneness. Cover. Let rest 5 minutes.
Meanwhile, for dressing, in a small bowl whisk together lime juice, remaining fish sauce, remaining sugar, 1 tablespoon water, and red pepper. In a large bowl combine noodles, lettuce, cucumber, carrots, basil, and mint. Thinly slice steak against the grain; arrange on salad. Sprinkle with peanuts. Serve with dressing and remaining lime, cut into wedges.