Preheat oven to 350 degrees. Boil potatoes in skins until about half done. Drain water from potatoes, remove skins, and cut in lengthwise slices. Place slices in a greased baking dish and dot with margarine. Make a syrup by boiling together brown sugar, water, and vanilla. Pour over potato slices. Bake 40-45 minutes, basting frequently with syrup. Cook until potatoes are transparent.