In a medium bowl, cover the vermicelli in cold water and soak for 30 minutes.

Meanwhile, in a mortar, pound the red Thai chiles and garlic cloves to a paste with 1 tablespoon of the sugar. Add the lime juice, fish sauce, boiling water, and the remaining 3 tablespoons of sugar and pound until the sugar is dissolved. Let the dressing stand for 30 minutes.

Bring a large saucepan of water to a boil. Fill a bowl with ice water. Add the shrimp to the boiling water and cook until white throughout and curled, 2 to 3 minutes. Using a slotted spoon, transfer the shrimp to the ice water. Add the scallops to the boiling water and cook until white and firm, 2 to 3 minutes. Transfer the scallops to the ice water. Add the squid to the boiling water and cook just until firm, about 45 seconds. Transfer the squid to the ice water. Drain all of the seafood and pat dry.

Bring a fresh saucepan of water to a boil and refill the bowl with ice water. Drain the vermicelli, add to the boiling water, and cook just until al dente, about 1 minute. Drain and transfer to the ice water. Drain again and pat dry. Cut the vermicelli into 3-inch lengths.

In a large bowl, toss the seafood with the vermicelli, tomatoes, bean sprouts, mint, red onion, peanuts, and chile dressing. Line a platter with the lettuce leaves and top with the seafood salad. Garnish with cilantro and serve.

All of the components can be prepared up to 1 day ahead and refrigerated separately. Toss the salad just before serving.