1. Preheat the oven on broiler setting.
2. In a large bowl, combine the roma tomatoes, sun-dried tomatoes, garlic, olive oil, vinegar, basil, salt, and pepper. Allow the mixture to sit for 10 minutes.
3. Cut the baguette into 3/4-inch slices. On a baking sheet, arrange the baguette slices in a single layer. Broil for 1 to 2 minutes, until slightly brown.
4. Divide the tomato mixture evenly over the baguette slices. Top the slices with mozzarella cheese.
5. Broil for 5 minutes, or until the cheese is melted.