Make sauce: Heat peanut oil in a small saucepan over moderate heat. Once hot, add onion, garlic, and red pepper flakes and cook, stirring, until pale golden, about 4 minutes. Whisk in water, peanut butter, hoisin, tomato paste, and sugar and bring to a simmer. Whisk for 1 minute, then transfer to a small bowl to cool.

Prepare noodles: Cook noodles according to package instructions. Rinse with cold water, drain well, and toss with seasoned rice vinegar.

Assemble spring rolls: Fill a large plate with warm water. Dip one rice paper wrapper into the water, completely submerging . Leave in for 5-10 seconds, until pliable. Lay wrapper flat on a clean work surface. Lay 2 shrimp halves, pink side-down, horizontally across the lower third of the wrapper.

Top with a sprinkling of lettuce, mint, Thai basil, cabbage, cilantro, carrot, and green onions. Add a small handful of the noodles on top, again in a horizontal line. Carefully fold the sides inward, then tuck and fold the bottom up over the filling. Roll everything upward, tightly but gently to avoid ripping the rice paper wrapper. Place on a platter and repeat with remaining ingredients.

Serve spring rolls with peanut sauce alongside, garnished with chopped peanuts if desired.