I like vests. I have a pair of fleece vests that I wear around town, to work and otherwise. But they never make it into my pack outside of winter, and I completely left them at home this past winter.
So what of vests for backcountry use? Looking at 3-season hiking, fleece is out - it's weight/warmth ratio cannot compete with puffy insulation. With down and synthetic insulation, there are many, many options out there. For down, you have the Feathered Friends Helios/Hyperion, Western Mountaineering Flash Vest, Mont-bell has two models and Patagonia has their sweater vests. If synthetic is what you need, there are Montbell Thermawraps, Patagonia Micropuffs and the BPL UL 60 Vest. Just to name a few.
But puffy vests present at least two problems with ultralight puffy jackets. Vests must be fully allowed to loft, but most ultralight puffy jackets are close-fitting and are not designed for layers other than base layers underneath them. Thus, you need to size up the jacket in order to wear the vest. This in turn decreases the thermal efficiency of the jacket, because it is larger, has more area to heat up and is likely baggier, especially on the sleeves.
The second problem is one of pure functionality versus weight. If your vest weighs 5.0 oz, and your parka is 7.4 oz, are you really going quibble with taking one over the other? And what happens when you need the jacket, but not the vest and taking both would be overkill? I can't imagine that the minimal weight gain outweighs loss of functionality in most cases. If you're taking both, well then that's another story. More on that below.
The analogy runs to jackets. As I have often said, I like hoods on all clothing that is meant as an outer layer - this means base layers, windshirts, softshells, hardshells and puffy insulation. Pieces that are strictly midlayers do not require hoods - the additional hood just gets in the way. So, if you don't need a piece as a midlayer, why get it without a hood? The thermal efficiency simply trumps any appreciable weight reduction.
So what's a hiker to do? I'd rather not buy two sizes of the same garment, one that allows layering under and one that doesn't. Hrm.
There are obviously exceptions to these arguments. First, I generally adhere to a principle of "layering-over." This layering philosophy means that I never take off a layer to add or remove a layer. Under this principle, one would never wear the puffy jacket without the vest underneath, and the problem is solved. So long as layering over each prior layer does not compromise any of your layers, the system works. This principle worked (almost) for me all of last winter. In the 3+ season trips I have applied it to, It has also worked. Vests were not involved in those trips, but could have been incorporated.
Second, winter is a different story, where you need puffy insulation to stay warm, but you don't want to turn into the little brother in Christmas Story. Here is where a puffy vest triumphs - it keeps your core warm, fills up empty space in your generally over-sized parka and allows you freedom of movement with your arms.
Third, wind vests could be nice when a wind layer is necessary but a full-on wind jacket is overkill. And so goes the exceptions. There are many, many more.
So, what think?