Pour the red wine vinegar into a small bowl, then gradually whisk in the olive oil. Season with salt and pepper.
Place the bread in a large bowl and pour in enough of the cold water to cover the bread.
Soak for about 5 minutes and drain well, squeezing the bread to remove as much liquid as you can.
Crumble the bread back into the same bowl, then add the tomatoes, onion, and basil.
Toss with the oil and vinegar mixture to coat, then season generously with salt and pepper.
This dish can be made up to 8 hours ahead of time and refrigerated.
Let it stand for a while at room temperature before serving.