To make the dressing, in a medium bowl whisk the warm water with the sugar until thoroughly combined. Add the fish sauce, lime juice, rice vinegar, garlic and red pepper flakes, and whisk to combine. Cover and set aside at room temperature for 1 hour.
To make the salad, bring a medium saucepan of water to a boil. Add the asparagus and cook until fork tender, 2 to 3 minutes. Drain and refresh in a bowl of ice water. Drain.
In a bowl, cover the noodles with hot water and soak until softened, about 3 minutes. Drain the noodles and cook in the boiling water until tender, 1 minute. Drain noodles and refresh in cold water. Drain.
In a bowl, toss the noodles with 1/2 cup of the dressing. Divide among 4 plates. Add the asparagus, crabmeat, and green onions to the bowl, and toss with 1/4 cup of the dressing. Arrange on top of the noodles. Garnish each serving with cilantro sprigs, chopped peanuts, and a drizzle of the remaining dressing.
Yield: 4 servings