Similar to a salad that has been rolled up, this dish is usually eaten as a snack, although it also makes a lovely lunch. The key is to make the rolls tight, and that requires practice. You can substitute chicken, beef, or tofu and mushrooms for the filling. Grilled fish such as salmon also works well. You can serve whole or cut into smaller pieces to make them easier to serve and share. The recipe calls for untrimmed pork because the dish benefits from a little fat.
Cook the pork in boiling salted water until done but still firm enough for slicing, about 30 minutes.
Meanwhile, bring another small pot of water to a boil. Add shrimp and cook until they turn pink, about 3 minutes. Rinse under running water and set aside to drain. When they're cool enough to handle, shell, de-vein, and cut in half lengthwise. Refresh in cold water and set aside.
Remove pork from heat and drain. When cool enough to handle, slice into thin slices, about 1 by 2 1/2 inches. Place on a small plate and set aside.
Set up a salad roll "station": Line a cutting board with a damp kitchen towel. Fill a large mixing bowl with hot water and place nearby. (Keep some boiling water handy to add to the bowl.) Arrange the ingredients in the order they will be used: pork, shrimp, rice vermicelli, bean sprouts, mint, and lettuce.
Working with 2 rice paper sheets at a time, dip 1 sheet, edge first, in the hot water and turn to wet completely, about 10 seconds. Lay it on the towel. Repeat with the second sheet and place it alongside the first. This allows you to work with one while the second is setting.
Line the bottom third of the rice sheet with 3 shrimp halves, cut side up, then top with two slices of pork. Add 1 tablespoon rice vermicelli, 1 tablespoon bean sprouts, and 4 to 5 mint leaves. (Arrange the ingredients so the rolls end up being about 5 inches long and 1 inch wide.) Halve a lettuce leaf lengthwise along its center rib. Roll up in one piece and place on the filling. (Trim if too long.) While pressing down on the ingredients, fold over the filling, then fold in the two sides and roll into a cylinder. If paper feels thick, stop at three-quarters of the way and trim the end piece. (Too much rice paper can make the rolls chewy.) Repeat with the remaining rice papers and filling.
To serve, cut rolls into 2 or 4 pieces and place them upright on a plate.