Halve shrimp lengthwise. Soak rice-paper wrapper in hot water until softened, about 5 seconds. On bottom of wrapper, place 3 shrimp halves, a Boston lettuce leaf, 1/4 cup cooked rice vermicelli, 1 tbsp. sliced scallions and 4 cucumber matchsticks. Fold in sides of wrapper; roll up. Repeat 3 times; serve with cocktail sauce.