Preheat the oven to 400 degrees F. In a large skillet, heat 2 tablespoons olive oil over medium heat. Cook sausage in the hot oil until lightly browned, about 2 minutes per side. Set aside. Place potatoes, onion, bell peppers, mushrooms, zucchini, grapes, and parsley in a large bowl. Season with salt, pepper, paprika, and remaining oil. Mix well. Spread vegetables on a rimmed baking pan lined with aluminum foil. Nestle the browned sausages into the vegetables. Add to the oven, and roast for 30 minutes, stirring halfway through. Remove from oven and set sausages aside. If necessary, season the vegetables with salt and pepper and lay out on a platter. Top with sausages (sliced or whole), remaining 2 tablespoons of parsley, and a drizzle of olive oil.