In a mixing bowl, combine the minced lemongrass, 2 tablespoons of fish sauce, vegetable oil, brown sugar, and minced garlic. Add the sliced chicken thighs and toss to coat thoroughly. Let marinate for at least 15 minutes.

Bring a large pot of water to a boil. Cook the rice vermicelli noodles according to package instructions (usually 3-5 minutes). Drain and rinse immediately under cold running water to stop the cooking process and prevent sticking.

Heat a large skillet or grill pan over medium-high heat. Add the marinated chicken pieces and cook for 5-7 minutes, turning occasionally, until the chicken is browned, slightly charred, and cooked through.

While the chicken cooks, prepare the dressing. In a small bowl, whisk together the lime juice, remaining 3 tablespoons of fish sauce, white sugar, and sliced chili until the sugar dissolves.

To assemble the bowls, place a portion of the cold noodles at the bottom. Arrange the cooked chicken, cucumber, carrots, mint, and cilantro on top.

Garnish with chopped peanuts and serve immediately with the dressing poured over the top.