Make the pickled vegetables: Put carrot and daikon in a small bowl and sprinkle with sugar and salt. Add rice vinegar, toss well, and set aside.
Make the dipping sauce: In a small bowl, stir together brown sugar, vinegar, lime juice, fish sauce, and serrano pepper. Stir in ½ cup cold water and let mixture sit for 15 minutes. (Leftover sauce will keep up to 3 days, refrigerated.)
Marinate the shrimp: Put shrimp in a shallow dish. Add fish sauce, brown sugar, garlic, and lemongrass. Mix well to coat. If you are going to grill, place the shrimp on skewers.
Meanwhile, bring a large pot of water to boil. Turn off heat and add rice noodles. Soak noodles, stirring occasionally, until softened, usually about 7 to 8 minutes. Drain and rinse with cold water. Leave in colander at room temperature.
Shred lettuce leaves and herb sprigs, place at the bottom of each bowl. Keep cool, covered with a damp towel.
Put oil in a wok or frying pan over high heat. When oil is hot, add shrimp without crowding (work in batches if necessary). Cook for about 2 minutes per side, until lightly browned.
To serve, divide the noodles among the 4 large soup bowls, then top each with hot shrimp, pickled vegetables and a tablespoon or so of dipping sauce. Sprinkle with scallions and peanuts (and beans sprouts if using). Serve with extra herbs, lime, and Sriracha.
For Vegan Option:Use tofu instead of shrimp. Replace fish sauce with coconut aminos.Alternatively:Use chicken or thinly sliced pork loin instead of the shrimp.