Combine vinegar, fish sauce, sugar, garlic, and pepper flakes in a small saucepan. Bring to a simmer and cook, whisking, until sugar dissolves, about 3 minutes. Transfer to a bowl and cool to room temperature for dipping sauce.

Bring to medium saucepan of water to a boil; add salt. Add noodles and cook, stirring, 40 seconds. Immediately drain in a colander and rinse with cold water. Drain well and transfer to a bowl. Toss with 2 tablespoons dipping sauce and snip into pieces about 4 inches long.

To assemble rolls, place noodles, shrimp, mango, cucumber, mint, cilantro, and chives in separate bowls. Working with 2 wrappers at a time, soak in a bowl of very warm water (gently moving so they don't stick) just until softened, about 30 seconds. Carefully remove each wrapper from water, allowing excess water to drip off, and arrange flat (with widest side nearest you, if a triangular shape) on a clean dry work surface. Leaving a 1/2-inch border on edge closest to you, mound a small amount of noodles, a couple of pieces of mango and cucumber and some mint and cilantro. Fold over border and begin rolling away from you until filling is enclosed, fold in side edges and continue rolling until 2 inches from end of wrapper, then top middle of roll with halved shrimp, pink side up, and 2 chives. Continue to roll wrapper until enclosed completely. Transfer roll to damp paper towels and keep covered with more damp paper towels to prevent rice pepper from drying out. Repeat process with remaining wrappers and filling. Arrange rolls on a platter and serve with dipping sauce on the side. Makes 36 rolls.