In a large, heavy-bottomed pan, combine 3 1/2 cups cranberries and 1/4 cup dark brown sugar. Split the vanilla bean pod lengthwise and use the tip of a sharp knife to scrape the vanilla beans from the pod and into the pot containing the fruit and sugar. Add the vanilla bean pod, and cook the mixture over medium heat, stirring frequently until the cranberries have released their juices, thickening and darkening in color, around 15 minutes.