Heat the oven to 425°F and toss the sweet potatoes and red onion with 1 tablespoon olive oil, maple syrup, rosemary, garlic powder and 1 teaspoon salt.
Spread the vegetables in a 9-by-13-inch baking dish and roast for 15 minutes.
Season the chicken thighs with the remaining salt and black pepper and rub them with the remaining oil.
Stir the vegetables, arrange the chicken over them and roast until the thighs reach at least 165°F and the potatoes are tender, 25 to 30 minutes.
Drizzle the cider vinegar over the hot vegetables and scrape up the browned juices.
Rest the chicken for 5 minutes before serving it over the sweet potatoes.