To prepare rolls: Cook shrimp in boiling water just until curled and opaque in the center, 1 to 2 minutes. Drain and refresh under cold running water. Transfer to a clean cutting board to cool, then chop.
Combine the shrimp with sprouts, scallions, mint, 1 tablespoon fish sauce and 1 tablespoon lime juice in a large bowl. Working with one wrapper at a time, soak in a large bowl of very warm water until softened, about 30 seconds. Place the soaked wrapper on a clean, damp kitchen towel. Put a generous 1/3 cup of the shrimp filling in the center, fold the wrapper over the filling and roll into a tight cylinder, folding in the sides as you go. Repeat with the remaining wrappers and filling.
To prepare dipping sauce: Stir together 3 tablespoons lime juice, water, 2 tablespoons fish sauce, sugar, garlic, serrano (or crushed red pepper) and scallion (if using) in a small bowl until the sugar is dissolved.
To serve, cut each roll in half and serve with the dipping sauce.