Cut mango flesh into strips 1/2 inch by 2 1/2 inches. Cut lettuce leaves in half, discarding center ribs. Trim each scallion into two 5-inch lengths; slice lengthwise into very thin strips.

Bring a pot of water to a boil. Add salt and rice vermicelli to boiling water, and cook until tender, 2 to 3 minutes. Drain, and rinse in cold water.

Transfer rice paper to a plastic bag. Place two paper towels, one on top of the other, on a work surface; dampen them slightly. Lay a sheet of rice paper on top. Cover with two more damp paper towels. Removing one sheet or rice paper at a time, repeat until you have ten layers. Let stand until pliable, 10 to 15 minutes. Meanwhile, line a baking sheet with plastic wrap.

Across the bottom third of one sheet of rice paper place two pieces of lettuce; four cilantro leaves; four mint leaves; one piece of crab (if using lump crabmeat, 2 tablespoons); two pieces of mango, end to end; and 1/4 cup vermicelli. Cover with four strips of scallion, four cilantro leaves, and four mint leaves. Roll rice paper into a cylinder, stopping halfway. Fold left and right sides into the middle; finish rolling.