While making a new down quilt/comforter I went nuts and decided to modify it to turn into a practical camp vest for lounging around on crisp evenings.
I will confess right here to contributing more than my share to the Bad-Ideas-Are-Us lore -- not to prejudice anyone against the following.
Having done the same thing with a synthetic quilt and having used it for a few hundred miles on the AT, I knew it might work to eliminate having to carry extra clothing for warmth around camp. So I wasn't shooting blind.
So imagine a 25 F down quilt with a zippered foot (30" #3 zipper) and zipped arm slits (18" #3) and a separating zipper from waist to collar (28"). The tops of the shoulders are closed and baffled and it has a down-filled collar that works front and back (since the back of the vest is the front or top of the quilt), and the arm and foot zips are backed by draft tubes. Weight came to 26 oz. - maybe 6-8 ounces more than I'm used to for a quilt this thick. (I'm having to guess here because no two of my projects are identical.) So it probably isn't much lighter than the combined weight of a thinner quilt rated for higher temperatures and a separate vest. However, it makes a very very warm vest, so that compensates somewhat. Of course, the bottom dragged behind, so I put a loop of waistband elastic on the foot to loop over a shoulder when wandering around camp. Sitting around, I use the bottom as a leg cover. I can unzip an arm to reach stuff when snugged down on the sleeping pad. So far, so good.
Construction was a little trickier than making a simple quilt, and the arm zip slits meant a more tedious down stuffing job since they interrupted the down chambers, but anyone who has made a quilt before ought to be able to figure out how to do this with either a down or synthetic filled quilt.
Things I've learned to avoid after making and using two of these chimera: #3 zippers are adequate for the foot when & only when backed up with snaps at the top and about 1/2 way to the foot. Otherwise, the non-locking zipper pulls wouldn't stay put. Locking pulls would fix that, but no one had them. Heavier zippers add up fast. Velcro works for the foot and arm holes if you don't mind the fiddle factor - and we all know about the farces that ensue when using Velcro. The non-sticking velcro I've been able to get so far doesn't work very well.
I still suspect that a separate vest that could be worn in bed to supplement a lighter quilt might, might, be lighter and would certainly be more versatile. In other words, I'm not sold, but I don't yet have enough data to give anyone definitive advice.