Boil the Pork: Bring water to a boil with salt and shallots. Add pork belly and cook until tender. Let rest 20 minutes, then slice thinly.
Boil the Shrimp: In a new pot of salted, shallot-infused water, cook shrimp just until pink. Peel and slice each shrimp in half lengthwise.
Cook the Noodles: Boil "Bún" for 9 minutes. Rinse in cold water and drain completely to prevent soggy rolls.
Make the Sauce: Sauté garlic and shallots in oil. Add pork broth, hoisin, and peanut butter; simmer until smooth. Finish with rice vinegar and crushed peanuts.
Assembly: Quickly dip rice paper in lukewarm water (1 second) and place bumpy side up. Layer in three rows:
Row 1: Lettuce and herbs.
Row 2: Pork belly and vermicelli.
Row 3: Shrimp (cut side down).
Roll: Fold the sides like an envelope, then roll tightly away from you until sealed.