Re: the VB - your body moisture goes thru the quilt. The silnylon quilt protector is a VB that would trap the moisture inside your quilt, wetting your down. But if you put a VB between your body and quilt, it stops the moisture from getting into your quilt in the first place.
Re: the DWR - all the current models of commercial quilts are made from water resistant fabric, which should be enough to protect them from moisture in normal use. Adding an extra layer of protection can make it easier to keep them dry...shaking droplets off a sil undercover is easier than shaking droplets off a DWR quilt, for example. I've experimented with undercovers and such and decided that it's not worth the pack space/weight for me, at least where I've hiked. Environment may change that at some point. In the end...if it makes you more comfortable and helps you sleep soundly at night b/c you're not worried about your quilts getting wet, then it's a good piece of gear for you!
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