In a bowl, stir lemongrass, garlic, soy and fish sauce, plus brown sugar until balanced. Add chicken thighs, coat evenly, cover, and chill for at least 10 minutes.

Warm oil in a skillet over medium-high heat. Sear each marinated thigh for about 6 minutes per side, until golden brown and cooked through (165°F internal temp).

Pour boiling water over rice vermicelli in a heatproof bowl. Let it soak until tender but still firm (about 5 minutes), then drain and rinse under cold water.

Slice cucumber into thin ribbons and julienne carrots. Toss both with torn mint and Thai basil leaves.

Distribute drained noodles and seasoned veggies across bowls. Slice chicken, then layer it atop each bowl.

Drizzle with lime juice and sprinkle peanuts, green onions, chili flakes, and cilantro.