Whisk together fish sauce, sugar, lime juice, garlic, red pepper flakes and, last, water. 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.