Bring a medium saucepan of water to a rolling boil over high heat. Drop in the rice noodles, and remove from the heat. Let stand 8 to 10 minutes, gently lifting and stirring the noodles now and then as they soften to keep them separate and to cook them evenly. Drain, rinse with cold water, drain well, and set a side. You should have about 2 cups of noodles.
Arrange all the ingredients in separate dishes around a large cutting board or tray set before you. Have a large platter ready to hold the finished rolls, and fill a large skillet or shallow bowl with hot water.
To make each roll, slide 1 sheet of rice paper into the pan of water and press gently to submerge it for about 15 seconds. Remove it carefully, draining the water, and place it before you on the cutting board.
On the bottom third of the sheet, line up the following ingredients in a horizontal row: a small tangle of noodles (about ½ cup), some lettuce strips, some mint leaves, and some cilantro leaves. Sprinkle green onion slivers on top.
Lift the wrapper edge nearest to you and roll it up and over the filling, tucking it in under them about halfway along the wrapper and compressing everything gently into a cylindrical shape. When you’ve completely enclosed the filling in one good turn, fold in the sides tightly, as though making an envelope. Then place 2 shrimp halves, pink side down, on the rice sheet just above the cylinder. Continue rolling up the wrapper and press the seam to close it, wetting it with a little splash of water if it has dried out too much to seal itself closed. Set the roll aside on the platter to dry, seam side down. Continue to fill and roll up the rice paper sheets until you have made 8 to 10 rolls. Set aside.
To serve, present the rolls whole, or cut them in half crosswise—straight or on the diagonal. Or trim away the ends and cut into bite-sized lengths.