JRB has separate, add-on down sleeves to make a vest into a jacket or to wear when you are using one of their wearable bags as a insulated poncho. Currently, the long sized sleeves in bright green are on sale for even less than the Xmas sale price on regular size. That is a good price on a good product.
My feet stayed warm all night on my last trip (23 degrees) only because the Jacks' sleeve instructions mentioned using them over your feet if needed. My thanks to Jack and Jack for that tip.