Combine vinegar, soy sauce, ginger, and garlic powder in a jar; cover tightly, and shake vigorously.
Wash snow peas; remove ends. Arrange snow peas in a vegetable steamer over boiling water. Cover and steam 3 minutes; drain and rinse with cold water.
Combine snow peas, water chestnuts, and parsley in a small bowl. Pour vinegar mixture over snow pea mixture; toss gently. Garnish with green onion fans, if desired.