Separate onion slices into rings. In a small bowl, combine onion, vinegar and oil; toss to mix. Season with salt and pepper. Marinate at room temperature for 2 hours. Remove onion rings and place them in a single layer on a nonstick drying sheet. Reserve any marinating juices for garnish. Place in dehydrator set at 105 degrees until tender and still slightly moist, about 2 1/2 hours. Remove, and set aside.