Reflective, high-viz gilet

[VeloVT]

The Pearl Izumi Optik vest in screaming yellow is about as bright and as lightweight as it gets.

It's much lighter and less bulky than either the PI Whisper or the Brooks vest. I think it breathes pretty well.

It runs pretty small.

I have the Optik jacket in two colors, and my boyfriend has the Optik vest in screaming yellow (which is indeed very bright). I use my jackets quite a bit actually, they are very light and reasonably breathable, and very packable. Be forwarned though -- the fabric is loud and makes quite a bit of rustling noise when you are riding downhill or into wind (and this is even when they are sized properly and have a fairly slim fit). One windy day I was riding with my boyfriend -- he was wearing the Optik vest, and I was wearing the Sugoi vest. The difference in noisiness was quite noticeable if we rode side-by-side. This is just an FYI -- depending on how sensitive you are to that kind of thing, it may not bother you at all. It doesn't really bother me when I wear my Optik jacket (although I notice it), but it drives my boyfriend a little nuts with the vest.

Edit: I agree with Mariposa on the sizing. In fact, I almost always have to size up with PI jackets -- I'm a small in their jerseys but definitely a medium in the jackets. They tend to run narrow through the shoulders/back.