Wash cucumbers, cut in half lengthwise, seed, and dry with paper towel; chill. Clean and stem mushrooms. Clean and hollow out cherry tomatoes, invert and chill. Cut potatoes into tiny cubes.
In a large serving bowl, mix yogurt, mayonnaise, vinegar, dill, salt and pepper until blended. Add remaining ingredients; toss gently to mix and coat. Serve at room temperature or chill. Makes 6 one-cup servings.