In a medium bowl, combine the sliced chicken thighs with fish sauce, soy sauce, brown sugar, minced garlic, minced shallot, black pepper, and sesame oil (if using). Toss well to coat the chicken evenly. Let marinate at room temperature for at least 30 minutes, or cover and refrigerate for up to 4 hours for deeper flavor.

While the chicken marinates, prepare the Nuoc Cham dipping sauce. In a small bowl, dissolve the granulated sugar in the warm water. Stir in the fish sauce, fresh lime juice, and rice vinegar. Add the minced garlic and thinly sliced red chilies (if using). Stir well and taste, adjusting for your preferred balance of sweet, sour, salty, and spicy. Set aside.

Bring a large pot of water to a rolling boil. Add the dried rice vermicelli noodles and cook according to package instructions, typically 3-5 minutes, until they are tender but still have a slight bite (al dente). Immediately drain the noodles in a colander and rinse thoroughly with cold water to stop the cooking process and prevent them from sticking together. Drain very well and set aside.

Choose your preferred cooking method for the chicken: * **Grill:** Preheat an outdoor grill or grill pan to medium-high heat. Grill the marinated chicken slices for 3-4 minutes per side, or until cooked through with nice char marks. * **Air Fryer:** Preheat air fryer to 375°F (190°C). Arrange chicken in a single layer in the air fryer basket (cook in batches if necessary to avoid overcrowding). Air fry for 10-15 minutes, flipping halfway through, until the chicken is golden brown and cooked through. * **Pan-Fry:** Heat 1-2 tablespoons of cooking oil in a large skillet over medium-high heat. Add chicken in a single layer (cook in batches if needed). Cook for 3-4 minutes per side, until browned and cooked through. Remove cooked chicken from heat and let it rest for a few minutes before slicing into bite-sized pieces if not already cut.

Divide the rinsed and drained rice vermicelli noodles among 4 large serving bowls. Artfully arrange a generous bed of fresh mixed greens, julienned cucumber, shredded carrots, and bean sprouts around or next to the noodles in each bowl.

Top each noodle bowl with the warm, cooked chicken slices. Garnish generously with fresh mint leaves, cilantro, chopped roasted peanuts, and fried shallots (if using). Serve immediately with the Nuoc Cham dressing on the side, allowing each person to pour as much as desired over their bowl and toss all ingredients together before enjoying.