Puree the tomato sauce in a blender or with an immersion blender. Whisk in the vinegar and season with salt & pepper. Taste for balance and chill. Heat the oil in a deep fryer or deep pot to 350°F. Meanwhile, cut the zucchini into 3-inch lengths, then into 1/8-inch-thick slices, and finally into 1/8-inch-wide sticks. Set aside in a small bowl. Place the onions in another small bowl. Drain the rock shrimp well and place in yet another bowl. Pour 1/4 cup buttermilk each over the zucchini, onions, and shrimp. Toss each well. Put the rice coating in a large bowl. Drain the zucchini well and add to the coating bowl. Toss well, transfer to a sieve, and shake off the excess. Fry in small batches until crisp and pale gold, about 3 minutes. Transfer with a slotted spoon to paper towels to drain and place in a low oven to keep warm. Repeat the process with the onions: cook them for about 2 minutes, drain, and add to the zucchini in the oven. Finally repeat the process with the shrimp, cooking them for about 3 minutes. To serve, divide the tomato dipping sauce among 4 small bowls. Pile equal portions of shrimp, zucchini, and onions on each of 4 plates with a bowl of dipping sauce. Or, roll a sheet of waxed paper or doubled newspaper into a cone & place in a tall glass, such as a parfait glass. Fill with fritto misto and place the bowl of dipping sauce to the side of each serving. Serve immediately. | 
