Cut the toasted bread into 1-inch pieces (you should have 2 cups). Transfer to a portable container.
Add cucumber, red onion, tomatoes, and feta. Toss lightly. Sprinkle mixture with vinegar, oil, and pepper to taste. Toss again. Salad is best marinated, when tomato juices have run into the bread. Keep at room temperature up to 3 hours, or refrigerate up to 6 hours.