Combine vinegar and fennel seeds in medium bowl. Gradually whisk in oil. Season dressing with salt and pepper. Cut all peel and white pith from grapefruits. Cut grapefruits between membranes to release segments. Stir segments into dressing. Let stand at least 15 minutes and up to 1 hour. Toss radicchio, spinach and olives in bowl. Add grapefruit segments and dressing to coat. Season with salt and pepper.