Vests are great for keeping the core warm while venting well, but for some reason vendors seem to think of down vests as an outer layer and make them too bulky. The MontBell U.L. Down Inner series turns this on its head with the Snap Neck Vest, an ultralight (4.5oz!!!) piece that fits just perfectly under a shell.
The vest has no front zipper to compromise the insulation, just a small opening at the top with a couple of snaps. It goes on and off like a t-shirt quite handily. The cut of the DWR nylon shell is a tad boxy and there’s no collar or pockets, but overall it’s a brilliant alternative to fleece, adding a lot more insulation for less space and weight: it compresses to smaller than a 20oz Nalgene.
I was belaying ice climbs from a crevasse in Alaska this past weekend. When the sun disappeared over the lip the temperature dropped accordingly, but I just threw the Snap Neck vest on under my soft shell and all was toasty.