In The Complete Walker IV, Chip Rawlins writes about using a VBL half-sac, covering from his feet to his upper chest. According to Rawlins, it was made by Moonstone but is no longer available. It had an elastic band at the opening. He takes this to bed with him, and if he gets cold, he pulls on the VBL half-sac.
Has anyone tried this type of VBL method? Rawlins suggests that it extends the temperature rating of his sleeping bag from 10 to 20 degrees.