Light and breathable windvest?

Mont Bell does.

You might get one of the custom makers to make a vest for you.

I have been experimenting with a wind layer that goes on/off quickly over the pack, for those edge conditions when there is varying breeze/sunlight/hills sufficient to make me want to switch layers several times.

So far, I’ve been happiest with an apron or small poncho design. Not a huge rain poncho; just a small poncho to throw over my torso made of breathable fabric. The apron is similar but has no back (which is mostly protected by my pack). I’ve also had some success with a small AD poncho for a little extra warmth that I can quickly change while moving.

My experiments have been with cheap materials but a finished product in Airwave/HyperD/Argon/Momentum should weigh just a couple of ounces.

By the way, I have a few of the Mont Bell ultralight wind jackets, with hood and without. The performance, since they changed to the new more breathable fabric, is excellent. It blocks wind as you would expect, but even when working hard, I do not find an appreciable buildup of sweat on me or condensation inside the fabric. I think they got the balance right on. (As an aside, I find their hood a bit too large and with insufficient adjustments, but that’s a different thread.) A vest would obviously offer even greater ventilation. I am 5’11”, 190 pounds with a bit of a barrel chest and wide shoulders, and I’m quite happy with the US size large. I also have the EE Copperfield (good hood) and a variety of other light wind shells. The Mont Bell offers similar performance. I would not hesitate to buy the vest.