Place the bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain bacon slices on paper towels and crumble.
Whisk mayonnaise, vinegar, and sugar in a bowl; refrigerate until ready to combine with salad.
Combine broccoli, onion, bacon, sunflower seeds, and cranberries in a large bowl. Drizzle mayonnaise dressing over broccoli mixture; toss to coat.