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