Cook the rice noodles as the label directs, then drain. Cut the cucumber and scallions into matchsticks and toss with 1 teaspoon salt in a bowl. One at a time, soak a rice paper wrapper in warm water until pliable, 30 seconds, then lay on a cutting board and pat dry. Arrange a couple shrimp across the middle of the wrapper; top with some noodles, herbs, cucumber, scallions and lettuce. Fold the bottom of the wrapper over the filling, then fold in the sides and roll up into a tight bundle. Cut in half; serve with sweet chili.