Heat grill to medium-high.
Cut potatoes into 1/2 inch thick rounds.
Cut the onion into 1/2 inch thick rings.
Toss potatoes and onions with the vinegar, 1 tablespoon of the oil, and ½ teaspoon each salt and pepper.
Divide the potatoes and onions between 2 large pieces of heavy-duty foil.
Fold the foil over the potatoes and seal the edges to form 2 packets.
Place on grill and cook, turning once, until the potatoes are tender, 20 to 25 minutes.