Place carrots and cucumbers in separate bowls; toss each with 2 to 3 Tbsp. of the sweet and sour sauce. Set aside.
Cook noodles in boiling water until tender, 2 to 3 minutes; drain. Rinse in cold water; drain very well. Transfer to a medium bowl; toss with 1/4 cup of the sauce. Set aside.
Cook prawns in boiling water until pink and translucent, about 2 minutes; drain. Cool; cut in half down the back. Transfer to a bowl over ice; toss in 1 Tbsp. of the sauce. Cover work surface with a clean, damp towel. Fill a 10-inch pie dish halfway with hot water (replenish as the water cools). Submerge 1 wrapper (smooth side down) at a time in the hot water. Let stand until almost soft, about 1 minute; transfer, smooth side down, to the towel.
Fill one wrapper at a time starting about 1½ inches in from sides and bottom edges, arrange herbs, basil leaves, several cilantro leaves, and 1 mint leaf across the bottom. Top with about 1/4 cup noodles and 1/15th of the carrots and cucumbers. Add a basil leaf and a couple cilantro leaves. Fold bottom edge up, overfilling to enclose it; bring the right and left edges up over filling. From the bottom, tightly roll once then line 3 or 4 prawns, pink side down, in a line parallel and close to the filling. Continue rolling into a tight cylinder; dab water on the end flap to seal. Place seam down on damp towel. Cover with a damp towel and plastic wrap.
Can be made 5 hours ahead (wrap in a damp towel and plastic wrap, refrigerate). To eat, wrap in a lettuce leaf, if desired, and drizzle with the sauce.
For Fiery Sweet and Sour Sauce: in a 2-quart saucepan, bring all ingredients, except the lime, to a boil, stirring to dissolve sugar. When mixture boils, immediately STOP stirring and reduce temperature to medium-low (keep it bubbling). It will boil over if the temperature is too high. Cook until slightly syrupy, about 15 minutes. Cool to room temperature; pour into a blender and liquefy (if mixture is hot, it may explode out of the blender).
Transfer to a serving bowl; stir in lime zest and 2 Tbsp. of the juice.