In a small bowl stir together the vinegar, the minced mint, and the sugar, let the mixture stand for 30 minutes, and force it through a fine sieve set over a bowl, pressing hard on the solids. Arrange 3 wedges of the melon on each of 4 chilled plates and drizzle each serving with some of the vinegar mixture. Drape each serving of melon with a slice of the prosciutto and garnish the plates with the mint sprigs.