1. Thinly slice avocado and arrange slices along one side of a wide, shallow bowl; brush lightly with vinegar. Mound lettuce in center of bowl, and arrange tomatoes opposite the avocado.
2. In a small bowl, mix 3 tablespoons vinegar, olive oil, and 1/2 teaspoon salt.
3. Pour dressing over salad; mix gently at the table, adding pepper and more salt to taste.