In a mixing bowl, combine fish sauce, soy sauce, brown sugar, vegetable oil, and lime juice to create the marinade. Add chicken thighs, coat well, cover, and refrigerate for at least 30 minutes.

Prepare the Nuoc Cham dressing by dissolving caster sugar in warm water, then stir in fish sauce, lime juice, minced garlic, and chopped chilli. Adjust taste as needed.

Bring a pot of water to boil, cook the noodles according to package instructions (4-5 minutes), drain, and rinse under cold water. Prepare vegetables by shredding lettuce and julienning carrot and cucumber.

Preheat grill or non-stick pan over medium-high heat. Grill or pan-fry marinated chicken thighs for 8-10 minutes until cooked through. Let rest before slicing.

In bowls, layer shredded lettuce, cooked noodles, julienned vegetables, sliced chicken, and top with peanuts and drizzle with Nuoc Cham dressing.