Combine cider vinegar, 1/2 cup water, and teaspoon sugar, stirring until sugar dissolves. Add onion; let stand 30 minutes. Drain. Cook English peas in boiling water with 1 teaspoon salt for 2 minutes. Drain; rinse peas with cold water. Combine onion, peas, pea shoots, and sliced radishes in a large bowl. Combine olive oil, white wine vinegar, 1/4 teaspoon salt, and pepper. Drizzle oil mixture over pea mixture; toss gently.