In a bowl stir together fish sauce, vinegar, sugar, lime juice, garlic and pepper flakes until sugar is dissolved. Chill covered for 1 hour.
Shell and devein shrimp. In a large saucepan of salted boiling water, cook shrimp with salt and lemon 2 minutes, or until just barely cooked through. In a colander, drain shrimp and rinse under cold water to stop cooking. Halve shrimp lengthwise.
Halve cucumber lengthwise and seed. Cut cucumber halves and scallions crosswise into 2 1/2 inch pieces and cut pieces lengthwise into julienne strips.
In a bowl, soak noodles in hot water to cover 15 minutes to soften. While noodles are soaking, in a 6-quart kettle, bring 3 quarts salted water to a boil. Drain noodles in colander and cook in boiling water 45 seconds, or until just tender. Drain noodles in colander, rinse under cold water to stop cooking. Drain well and with scissors cut into 4-inch lengths.
Divide noodles, shrimp, vegetables and dressing among 4 bowls and sprinkle with herbs and peanuts. To eat, pull herbs from sprigs and toss with noodles.