Pikeur Ghia. It's a softshell, v lightweight & breathable & really not much hotter in the summer than riding in a polo shirt. I've also ridden in it in light rain & water just beads off of it. The styling meets the requirements you described. It's very expensive but can be bought much more inexpensively online from Europe. And it's really worth it! In my experience, the sizing is the same as the Diana.
I bought an Arista which is lovely. I have the navy. It is a micro fiber, 4 silver buttons (except the really snap and just look look buttons), double vent back and matching velvet on collar and pockets. It is unlined and is very flattering. I have the tall in medium and the sleeves are actually long enough. Plus it is machine washable and costs around $280.00. It also comes in black. Not alot of stores carry Arista, though I don't know why. I have not worn my show coat yet, but I also have their fitted zip-up clinic jacket in the same fabric and it is stunning. Worth the look even though you want a single vent.
Wits End is where mine came from! I searched all over for a medium tall in navy after trying on a black one at Dressage At Devon. I contacted the owner of Arista and she found the only one available at the end of last year at Wits End.
I don't know if you can get them in the States, but I have a Spooks jacket which I think is just beautiful, and it's definitely great in the heat. I've ridden in it in competitions in the middle of Australian summers, and felt relatively cool considering (it's difficult to honestly feel cool in the middle of summer, i guess!)
Another that I have my eye on, and would LOVE to snap up if I could ever find another stockist, is a Tattini jacket. BEAUTIFUL fit, lightweight, very stylish. I wish I'd snapped it up when I first saw it as I'm yet to find anywhere that stocks this brand here. Comes in navy or black with trim in a variety of colours (the navy with red trim is gorgeous) http://tattiniriding.co.uk/item/159