Soak the capers in 2 tablespoons of vinegar in a small bowl for 10 minutes. Drain.
Cut bread into 2-inch slices, evenly brush the slices with 2 tablespoons olive oil, distributing the oil among the slices and on both sides. Grill both sides of the bread in a grill pan until golden. Transfer bread to plate to cool briefly. Cut bread into 1-inch cubes and set aside.
Cut the tomatoes in half and scoop out the seeds. Cut the tomatoes into 1-inch cubes and set aside.
In a large bowl, whisk 2/3 cup of oil, 1/4 cup of vinegar, and garlic. Season with salt and pepper, to taste. Add the bread cubes, tomatoes, basil, capers, and red onion; toss to combine. Serve immediately.