Preparation Combine mayonnaise, 2 tablespoons vinegar, green onions, 3 tablespoons dill and buttermilk in small bowl; whisk to blend. Season with salt and pepper. Cover and chill at least 1 hour or up to 1 day. Combine remaining 4 tablespoons vinegar, 1 tablespoon dill and 1/2 teaspoon salt in medium bowl; mix in zucchini. Let stand at least 1 hour and up to 3 hours at room temperature, tossing occasionally. Drain thoroughly. Steam potatoes until just tender, about 12 minutes. Cool to just warm, about 10 minutes. Transfer to large bowl. Add sliced eggs, celery and zucchini. Mix in dressing; Season with salt and pepper. Sprinkle with grated egg. (Can be prepared 2 hours ahead. Cover and let stand at room temperature.)
Preparation Combine mayonnaise, 2 tablespoons vinegar, green onions, 3 tablespoons dill and buttermilk in small bowl; whisk to blend. Season with salt and pepper. Cover and chill at least 1 hour or up to 1 day. Combine remaining 4 tablespoons vinegar, 1 tablespoon dill and 1/2 teaspoon salt in medium bowl; mix in zucchini. Let stand at least 1 hour and up to 3 hours at room temperature, tossing occasionally. Drain thoroughly. Steam potatoes until just tender, about 12 minutes. Cool to just warm, about 10 minutes. Transfer to large bowl. Add sliced eggs, celery and zucchini. Mix in dressing; Season with salt and pepper. Sprinkle with grated egg. (Can be prepared 2 hours ahead. Cover and let stand at room temperature.)