Fyi, i used to own a Vireo. It really doesnt work well. In theory, the idea sounds cool (use your down jacket for upper, and Viero for bottom). The reality is that too much head loss at open end of bag. Also, wind hitting upper half is source of major heat loss too. I sold it and bought this bag (see below) with the same overall fill weight/size. My lesson learned: having an overall enclosure of body within one bag kept the body heat inside one space (plus other clothes and down can fit inside too. Good luck and kind regards!
http://www.westernmountaineering.com/in ... e%20Series