In a bowl, combine fish sauce, brown sugar, grated lemongrass, lime juice, and finely grated garlic to create a fragrant marinade. Add the finely sliced boneless chicken thighs, ensuring each piece is well coated. Let this mixture sit for 15 minutes at room temperature.

In a separate bowl, mix together peanut butter, hoisin sauce, white vinegar, and finely grated small garlic clove. Gradually add milk, stirring until you achieve the desired creamy consistency. Taste and tweak with brown sugar and salt to your liking.

Bring a pot of water to a boil, then remove it from heat. Place the dried vermicelli noodles into the hot water and let them soak for about 2 minutes until softened. Drain and rinse under cold water.

Heat 1 tablespoon of canola oil in a medium skillet over medium-high heat. Add half of the marinated chicken, cooking for about 3 minutes until browned. Repeat with the second half.

Fill a large, shallow dish with warm water. One at a time, submerge a rice paper sheet for about 2 seconds until flexible. Transfer to a clean cutting board with the smooth side facing up.

Place a small handful of soaked vermicelli noodles, cooked chicken, green cabbage, red cabbage, julienned carrot, chopped coriander, and mint across the center of rice paper. Fold the sides and roll tightly from the bottom up.

Arrange the rolled rice paper rolls on a platter. Serve with the creamy peanut dipping sauce.