In a large bowl whisk together yogurt, mayonnaise, vinegar, parsley, onion and herbs. Cook pasta until tender, about 12 minutes. Drain pasta and rinse with cold water. Drain and toss with dresssing and salt and pepper to taste. Salad may be made 4 hours ahead and chilled, covered. If pasta absorbs dressing while standing, toss with about 3 tbsp warm water.