Soak noodles in a pot of water while you stir together the nuoc cham. Stir dressing until sugar is dissolved.
Set the noodle pot over moderate heat and bring to boil. Cook just until noodles are flexible, about 2 to 3 minutes—refer to package instructions. Drain in a colander and rinse with cold water. Transfer to a bowl that will fit in your refrigerator. Drizzle the nuoc cham over the noodles, toss to evenly coat and refrigerate overnight.
The next day just before serving, tear herb leaves and scatter over noodles. Add julienned vegetables and sliced jalapeño. Combine gently, squeeze with half a lime, and serve with cooked marinated tofu, shrimp, or chicken.