Toss the onions with 1/2 tablespoon of the oil on a large rimmed baking sheet. Arrange the chicken thighs on top and sprinkle them all over with the smoked paprika and salt and pepper. Drizzle the vinegar onto the baking sheet, then roast in the oven about 15 minutes.
Meanwhile, toss the cherry tomatoes, olives, rosemary and garlic with the remaining 1 tablespoon oil, a big pinch of salt and pepper to taste. Add the mixture to the baking sheet with the roasted chicken and continue to roast until the chicken is golden brown and cooked through, 10 to 15 minutes more.
Sprinkle with the parsley and serve the chicken with the vegetables and juices from the baking sheet.