It's not particularly lightweight if you're trying to do a 1:1 swap, but might save more weight if you're creative, possibly by using it as part of your pad cushioning and sleep insulation. Since I generally sleep on my side, I've thought about getting one of these to take some of the pressure off my shoulder.
If you use the link below to make your first purchase, my account will eventually be credited $10. You don't have to use my link, but you do have to be invited to join the site.
Klymit Red Rock Kinetic Vest With Dry Air Pump