Whisk olive oil, vinegar, and garlic in small bowl to blend; season dressing to taste with salt and pepper. Layer prosciutto, mozzarella, tomatoes, and basil over bottom of bread. Drizzle lightly with dressing, then sprinkle with salt and pepper. Press top of bread over. Cut bread equally into 4 sandwiches. Prepare barbecue (medium heat). Grill sandwiches until bread is golden brown and cheese melts, pressing occasionally to compact with large spatula, about 5 minutes per side.