Adjust oven rack to upper-middle position and heat oven to 475 degrees. Toss cauliflower, carrots, bell pepper, onion, garlic, vinegar, and 1 T. oil in a large microwave safe bowl. Cover with plastic wrap and microwave on high power until vegetables are slightly softened, 2 to 5 minutes.
Pat chicken dry with paper towels, season with salt and pepper, and sprinkle with rosemary. Heat remaining oil in large oven-safe skillet over medium heat until just smoking. Cook chicken until well browned, about 5 minutes per side.
Remove chicken from pan and add vegetables to skillet. Arrange chicken, skin-side up, over vegetables and transfer to oven. Roast until chicken is cooked through and vegetables are tender, about 15 minutes. Serves 4.
