In a large bowl, combine the squash, red bell peppers, sweet potato, and Yukon Gold potatoes. Separate the red onion quarters into pieces, and add them to the mixture.
In a small bowl, stir together thyme, rosemary, olive oil, vinegar, salt, and pepper. Toss with vegetables until they are coated. Spread evenly on a large roasting pan.
Roast for 35 to 40 minutes in the preheated oven, stirring every 10 minutes, or until vegetables are cooked through and browned.