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