In a large saucepan, place the shrimp in 2 quarts of cold water. Add the bay leaf, peppercorns, and the halved and squeezed lemon. Bring to a boil over moderately high heat. Add the corn and remove from the heat. Drain and turn into a serving bowl. Remove and discard the halved lemon and bay leaf. Fold in the scallions and lemon slices.

Make the vinaigrette and pour over the corn and shrimp, tossing to coat. Let stand for 15 minutes. Season to taste with salt and serve warm.