Whisk together the vinegar, 2 teaspoons salt and some pepper in a small bowl. Gradually whisk in the oil, starting with a few drops and then adding the rest in a steady stream, to make a smooth dressing.
Shear off the corn kernels with a sharp knife over a bowl (you should have about 4 cups). Toss in the tomatoes, mozzarella and scallions. Pour the vinaigrette over the salad and toss to coat. Cover and let stand for at least 15 minutes and up to 2 hours. Before serving, tear the basil over the salad and stir.
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