An evaporative cooling vest not only keeps you cool but reduces dehydration. It works the same way as putting on a wet T shirt without having to deal with the "see through" part of it. Most have a lining on the inside that will keep your clothes dry but it still works very simply. Get it wet and put it on under a mesh jacket. The wind will go through the mesh jacket and evaporate the water from the vest, this will create a cooling process that keeps the vest cool to the touch.
The best way to keep it wet is to soak it in your cooler while you are off the bike. Make sure to read the instructions, each cooling vest manufacturer has different soak lengths. When you are ready to get back on the bike, the vests will be nice and cool. Since you won't sweat as much wearing the vest, you will help to reduce the chances of dehydration, but don't let this reduce the amount of water and fluids you drink. Ensuring you stay hydrated is still very important.
There are different designs of vests that are on the market. Most are a simple vest with Velcro enclosures. Joe Rocket sells their "Sahara Vest" that will zip into some of their jackets which can be convenient. The body of the vest is sewn in the same fashion as a quilt and trapping small gel beads inside that soak up the water. Some of the poorer quality vests can become slimy from the gel so look for good quality materials.
One other feature they have is color. These vests can double as a safety item so look for florescent greens and oranges. They'll be under your jacket most of the time so it won't look like some bad fashion statement. Besides, it's better to be seen anyway.
