In a bowl, combine the cauliflower, broccoli, carrots, zucchini and onion; set aside. In a saucepan over medium heat, bring vinegar, sugar and salt if desired to a boil. Remove from the heat; stir in oil. Pour over vegetables and toss to coat. Cover and refrigerate overnight. Just before serving, sprinkle with sunflower kernels if desired. Yield: 12-14 servings.