Cook and shred the chicken, then let cool completely. For the best texture, use two forks to pull the meat into thin strands.
In a small bowl, whisk together fish sauce, lime juice, rice vinegar, and brown sugar until the sugar dissolves completely.
Combine shredded chicken, cucumber, carrots, mint leaves, cilantro, and sliced red onion in a large bowl.
If using rice vermicelli noodles, rinse them under cold water, drain thoroughly, and gently mix them into the salad ingredients.
Pour the fish sauce mixture over the salad ingredients, then add the cilantro lime vinaigrette and toss everything gently to coat evenly.
Taste and adjust seasonings if needed—add more lime juice for tanginess, fish sauce for saltiness, or brown sugar for sweetness.
Sprinkle with chopped peanuts just before serving. If using, add the Thai basil leaves and crispy fried shallots at this stage as well.
Serve immediately for the freshest flavor and texture, or chill for 15 minutes to allow the flavors to meld together.
