Cut off tomato tops. Scoop out seeds and remove some dividing walls, leaving three or four large sections. Do not squeeze the tomatoes. Salt lightly and drain upside down on a platter.
Rinse shrimp. Bring 2 qt (1.9 L) water to a boil with vinegar and 1 tbsp (15 ml) salt. Add shrimp and cook for 1 minute after the water returns to a boil, or longer depending on size. Drain, shell, and devein, then cool completely.
Reserve 6 best-looking shrimp. Chop the rest (not too fine) and mix with mayonnaise, capers, and mustard.
Shake off excess liquid from tomatoes without squeezing. Fill to the top with shrimp mixture. Garnish each tomato with a whole shrimp and 1 or 2 parsley leaves. Serve at room temperature or just slightly chilled.
