Whisk together fish sauce, lime juice, sugar, red pepper flakes, and garlic. Set aside.

Place noodles in a large bowl in sink; cover them with hot water and let stand for 10 minutes. Meanwhile, bring a large pot of water to a boil. Drain noodles and add to pot; cook, stirring, until tender, 3 to 4 minutes. Drain and rinse with cold water until cool.

Transfer noodles to a large salad bowl. Add chicken and remaining ingredients; toss well. Pour half of dressing on salad and toss again. Add more dressing as desired.