1. Cook rice vermicelli noodles according to package directions. Drain and rinse with cold water; drain.
2. Combine 1/2 cup lukewarm water and granulated sugar in a medium bowl, stirring until sugar dissolves. Add lime juice, vinegar, fish sauce, and chiles; set aside.
3. Combine lettuce, cucumber, and herbs; set aside.
4. Combine cornstarch, brown sugar, salt, and pepper in a large bowl; stir until well combined. Add shrimp; toss to coat. Heat a wok or large skillet over high heat. Add 1 1/2 teaspoons oil, and swirl to coat. Add half of shrimp; cook for 1 1/2 minutes on each side or until shrimp are seared. Remove from pan. Add 1 1/2 teaspoons oil to wok; repeat procedure with remaining shrimp. Reduce heat to medium-high. Add remaining 1 tablespoon oil to wok; swirl to coat. Add onions and garlic; stir-fry 30 seconds. Return shrimp to pan; stir-fry 1 minute.
5. Arrange about 1 cup lettuce mixture in each of 4 large bowls, and top each serving with about 1 cup noodles and 2 tablespoons chopped peanuts. Divide the shrimp evenly among servings, and serve each with 1/4 cup sauce.