Heat oven to 400° F. In a 9-by-13-inch baking dish, combine the butter, sugar, vanilla, and 1/4 cup water. Place the dish in the heating oven until the butter melts. Remove from oven and stir to combine.
Cut each peach in half and remove the pits. Arrange the peach halves in the dish cut-side down. Roast until the peaches are tender, about 20 minutes. If desired, slip the peach skins off and discard.
Divide the warm peach halves among 6 bowls cut-side up, spoon the pan juices over the top, and serve with a scoop of the ice cream.