Whisk together vinegar, shallot, remaining 1/4 teaspoon salt, and remaining 1/8 teaspoon pepper in a small bowl, then add remaining 5 tablespoons oil in a slow stream, whisking until emulsified. Add half of dressing to cauliflower and toss to coat. Add romaine, radicchio, parsley, half of nuts, and remaining dressing to cauliflower and toss to coat. Season with salt and pepper and sprinkle with remaining nuts.