Combine 2 tablespoons fish sauce, sugar, and half the minced garlic in a bowl. Add the chicken thighs and turn to coat. Let them marinate for 15 minutes while you prepare the other components.

Bring a large pot of water to a boil. Add the rice vermicelli and cook according to package directions until just tender. Drain and rinse under cold water to stop the cooking. Set aside in a colander to drain fully.

Heat a grill pan or outdoor grill over medium-high heat. Cook the chicken thighs for 5 to 6 minutes per side until lightly charred and cooked through. Remove from heat and let rest for 5 minutes, then slice into thin strips.

Stir together the remaining fish sauce, lime juice, warm water, and remaining garlic to make the dressing. Taste and adjust the balance if needed.

In a large serving bowl, layer the shredded lettuce, cooked vermicelli, cucumber, and carrots. Top with the sliced chicken, mint, and cilantro.

Drizzle the dressing over the salad just before serving and sprinkle with chopped peanuts.