Whisk together the cider vinegar, sugar, mayonnaise, salt, and pepper in a medium bowl. Set aside.
In a large bowl, combine the broccoli with the onion, cranberries, and sunflower seeds. Pour the prepared dressing over top and mix well. Refrigerate for at least an hour.
Sprinkle on the cooked and crumbled bacon (or bacon bits), if using, right before serving to preserve the texture of the bacon.