For the dipping sauce: Put soybeans, onion, sugar, and vinegar into a blender, cover, and purée until smooth. With the motor still running, gradually add 3⁄4 cup water through hole in blender lid. Transfer to a small saucepan, bring to a boil, then simmer over medium-low heat until thick and reduced by about half, 10–15 minutes. Allow to cool.
For the roll: Cook pork in a pot of simmering salted water over medium heat until cooked through, about 30 minutes. Drain, transfer pork to a cutting board, and allow to cool. Cut pork into thin 1" × 2" slices and set aside. Bring a small pot of water to a boil, add shrimp, and cook until pink, 1–1 1⁄2 minutes. Drain and rinse under cold water until cool. Peel, devein, and halve shrimp lengthwise, then set aside. Bring another small pot of water to a boil, add noodles, and cook until softened, 2–5 minutes. Drain and rinse noodles under cold water until cool. Drain noodles well and set aside. Spread a clean damp dish towel out on a work surface. Fill a wide bowl with hot water and place it next to the towel.
For each roll, dip 1 rice paper wrapper into the hot water for 8–10 seconds, turning it to wet completely. Lay wrapper flat on the damp towel. Lay 3 pieces of shrimp, cut side up, along bottom third of wrapper, leaving about a 1" border on bottom and sides. Layer 2–3 slices of pork, a small handful of noodles, 6–8 bean sprouts, and 4–5 mint leaves on top of shrimp, in that order. Roll up 1 piece of lettuce lengthwise and put on top of mint. Using both hands, gently press down on filling and roll wrapper around filling once into a tight cylinder, folding sides of wrapper over filling halfway through. Trim off top unused part of wrapper. Set summer roll aside and cover with another damp towel.
Put dipping sauce into four or six small dishes (depending on the number of diners and add some chile paste and peanuts to each. Serve summer rolls on a plate with dipping sauce on the side. Garnish with lettuce leaves and branches of mint, if you like.