Before peeling grapefruit, zest about one-quarter of the rind with a grater or zester (you should have about 1 teaspoon finely grated zest). Set aside.
Peel grapefruit: Using a sharp knife, slice off both ends. Carefully slice downward, following the shape of the fruit to remove remaining rind and all pith. Slice grapefruit in half lengthwise, and place cut side down on a cutting board. Slice each half into six semicircles; set aside.
Place reserved grated zest in a small bowl, and add vinegar and salt, if using. Whisk in oil, and set aside.
Slice endive in half lengthwise, and remove core with a small knife. Slice each half lengthwise into 1/4-inch-wide strips. Place in a medium bowl, and add the salad greens; drizzle with dressing, and crumble in half of the goat cheese. Toss thoroughly to coat leaves evenly.
Divide among four salad plates, and arrange three grapefruit slices on top of each serving. Slice remaining goat cheese into four rounds, and place one on top of each salad. Season with pepper, and serve immediately.