Preheat oven to 350 degrees F (175 degrees C). In a small bowl combine stuffing and butter. In a separate bowl combine zucchini, carrots, onion, soup and sour cream. Mix in 1/2 of the stuffing mixture, then spread into a 9x12 inch casserole dish and sprinkle the remaining stuffing on top. Cover the dish with aluminum foil and bake in preheated oven for 60 minutes.