In a blender combine vinegar, mustard, honey, the 1/4 teaspoon salt, and some fresh ground black pepper. Cover and blend for 30 seconds. With the blender running slowly, add oil in a thin, steady stream. (If necessary, stop blender and scrape down sides.) In a salad bowl, combine greens, mango, goat cheese, and cranberries. Add mint sprigs, to liking; drizzle with dressing. Toss gently to coat. Season, to taste, with salt and pepper.