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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

I find a hooded puffy is key, that an cinching it up around your neck...

I'm a big fan of Feathered Friends, especially their down clothing. However, I also found one of their "specialty" bags, Winter Wren (25F), good in idea but not in reality. It simply isn't warm enough for the weight and the bulk. To allow for the arm openings, the sides are sewn through, not baffled. Also the main center zipper needs a second draft tube; I get a cold spot right down the chest, the worst possible place for me. And the hood is big and bulky, as is the leg section (which is necessary to allow the bag to be pulled up to waist and let you walk around).

The idea behind the bag is fine; the arm and leg openings are very useful, especially if you are using a bivy sack. The lighter version, Rock Wren, may work better for its temp rating (35F).

Anyways, if anyone wants to take a long Winter Wren (Epic shell) off of my hands (hardly used), I'll sell it for $200 +shipping.