Combine vinegar and sugar in a saucepan. Bring to a boil; cook 7 minutes. Scrape seeds from bean; place seeds and bean in pan. Stir in broth and Madeira; return to a boil. Cook 15 minutes, stirring frequently. Combine water, cornstarch, and salt; stir well, and add to pan. Bring to a boil, and cook 1 minute, stirring constantly; discard bean.