Remove roots, outer leaves, and tops from leeks, leaving 2 inches of each leek. Cut leeks into 2-inch pieces. Steam leeks, covered, 10 minutes or until tender. Rinse leeks under cold water; drain and chill. Combine vinegar, oil, mustard, salt, and pepper in a small bowl; stir with a whisk. Stir in capers. Divide leeks and tomatoes evenly among 4 small bowls. Spoon dressing over vegetables; sprinkle with basil.