Allow the salad to sit for a few minutes before serving to meld the flavors.
Whisk together vinegar, mustard, salt, sugar, and pepper in a large bowl. Add oil in a slow stream, whisking constantly until dressing is emulsified. Add tomatoes, mozzarella, and basil and toss well. Season with salt and pepper.
Cook's notes: Salad without vinaigrette can be assembled 2 hours ahead and kept at room temperature. Salad can be dressed 1 hour ahead and kept at room temperature, covered. Vinaigrette keeps, covered and chilled, 1 week.