Expect the noodles to clump and stick after they're cooked. Recipe author Andrea Nguyen devised a great trick to prevent them from sticking too much: Put an inverted small bowl in the bottom of the colander for the noodles to drape over.
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.