The combo of fish and mango is very popular in Thai cuisine.
Remove meat from crab legs. Slice in thin strips 5 inches long, cover, and refrigerate.
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 2 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 2 more damp paper towels. Removing 1 sheet of rice paper at a time, repeat until you have 10 layers. Let stand until pliable, 10 to 15 minutes. Meanwhile, line a baking sheet with plastic wrap.
Across the bottom third of 1 sheet of rice paper place 2 pieces of lettuce; 4 cilantro leaves; 4 mint leaves; 1 piece of crab (if using lump crabmeat, 2 tablespoons); 2 pieces of mango, end to end; and 1/4 cup vermicelli. Cover with 4 strips of scallion, 4 cilantro leaves, and 4 mint leaves. Roll rice paper into a cylinder, stopping halfway. Fold left and right sides into the middle; finish rolling.
Place on prepared baking sheet, cover, and refrigerate. Repeat entire process, making 10 rolls. To serve, trim ends with a sharp knife and cut the roll into 4 pieces. Serve dipping sauce on the side.
These rolls can be made several hours in advance and stored at room temperature in an airtight container lined with dampened cheesecloth. Do not refrigerate, or the rice paper will toughen.