Soak onion in cold water for 10 minutes. Squeeze onions and dry on paper towels – this will remove some of the sharpness. Drain beans and place in large bowl. Add corn kernels, cilantro and peppers. In a small bowl, whisk together vinegar, olive oil, cumin, salt and pepper. Pour over beans and corn and mix gently but thoroughly. Taste for seasoning. Refrigerate for two hours before serving.