Boil the rutabaga and potato until soft, about 25 minutes. Mash the rutabaga, using either a food processor or a potato masher.
Cut the vanilla bean in half lengthwise, scrape out the seed and sir into the mashed rutabaga.
The rutabaga can be left like this for up to 24 hours. Just before serving, heat the mashed rutabaga while stirring with a wooden spoon. Stir in butter and serve.