In a medium bowl, combine fish sauce, brown sugar, grated lemongrass, lime juice, and minced garlic to create a fragrant marinade. Add the finely sliced boneless chicken thighs, ensuring they are well-coated. Let the chicken marinate at room temperature for 15 minutes.

Bring a pot of water to a boil. Add the dried vermicelli noodles and let them soak for about 2 minutes until tender but firm. Drain the noodles, cool them under cold running water, and set aside in a bowl.

In a small bowl, whisk together the peanut butter, hoisin sauce, white vinegar, milk, minced garlic, brown sugar, salt, and optional chili paste until smooth. Adjust the consistency by adding more milk if necessary.

Heat a non-stick skillet over medium-high heat and add a drizzle of oil. Add half of the marinated chicken to the skillet, cooking for 3-4 minutes until caramelized. Remove and let cool before repeating with the remaining chicken.

Fill a large, shallow bowl with warm water. Dip one rice paper sheet into the water for about 2-3 seconds, making sure it becomes pliable but not too soft. Remove and let excess water drip off.

Position the hydrated rice paper sheet with the bottom edge facing you. Layer a small amount of coriander, pieces of cooked chicken, vermicelli noodles, julienned carrot, and finely sliced cabbages. Finish with fresh mint leaves.

Fold the bottom edge of the rice paper over the fillings, tucking them in snugly. Fold in the sides and continue rolling tightly until the end of the rice paper is reached.

Arrange the finished rolls on a platter. Pair with the peanut dipping sauce, either on the side or drizzled atop. Optionally, slice each roll in half for a nice presentation.