My main question is how cold will you be going out in? The temperature in NH routinely goes below 0F in the winter, although it can also go well above freezing, if it feels like it. I've personally been out in -13F, and have seen forecasts for much colder than that.
I'm not sure that I'd want to be out in a 10F bag when it hits -10F. In order to do that, the bag would have to be roomy enough to allow you to sleep with your normal down clothing either on you or layered over you, depending on taste. If you get a narrow bag for efficiency, then that won't work. My personal preference has been either a MontBell stretchy-something ( I have the discontinued UL Super Stretch series) or a Nunatak down quilt.
Yes, the VBL will add additional warmth, but not that much. Also, consider the possibility of using VBL clothing rather than a bag liner. If you sleep in down clothing, then the VBL liner will trap the moisture inside the clothing, and it will accumulate over a few days. And VBL is generally most effective when used near the skin.