In a mixing bowl, combine chicken thigh fillets with lemongrass, garlic, lime juice, fish sauce, soy sauce, brown sugar, and vegetable oil. Stir well and let marinate for at least 1 hour.

Heat a skillet or grill over medium-high heat. Add vegetable oil, then the marinated chicken, cooking for 6-8 minutes until golden brown and cooked through. Remove and rest before slicing.

For the Nuoc Cham sauce, whisk together fish sauce, rice vinegar, sugar, lime juice, minced garlic, and chili in a small bowl. Let sit for at least 20 minutes.

Soak the vermicelli noodles in a large bowl of hot water for 3-5 minutes until soft. Drain and rinse under cold water.

To serve, place noodles in bowls, top with sliced chicken and fresh vegetables, and drizzle with Nuoc Cham sauce.