Combine the yogurt and mayonnaise and set aside. Rinse and quarter the potatoes (no need to peel) and cook in boiling salted water until fork tender, 15-20 minutes. Drain and transfer the potatoes to a bowl and roughly cut into smaller pieces with a knife. Add the vinegar, olive oil, salt and pepper to the hot potatoes and gently toss. Add the onion, celery, yogurt/mayo mixture and mustard. Gently mix to combine. Fold in the eggs and parsley, if using. Chill in the refrigerator for at least 2 hours.