That's exactly what I did. My current one is the second version; I didn't like the first one so I knew what to do differently the second time around!
It's cotton sheeting, lightweight but with enough body to hold its shape. It's a green jungle print with birds; I cut the pockets so they each have a full bird on them. It's loose-fitting but has elastic at the waist so it won't flop all over. Front closes with three snap clasps (like the ones on backpacks) but with a gap, again to allow for airflow. Plenty of pockets, inside for money and valuables, and outside for phone, iPod, snacks, notebook and pen, etc. Back has a large pocket that can hold two 2-liter bottles of water or a camelback. The water provides a counterweight to keep the vest from riding up in the back when the front is full of stuff. I found the right position for the back pocket so I can reach it to get water bottles in or out without taking it off; I am thinking about a flap because if I bend over too far the bottles fall out, but that would complicate taking them out with the vest on. It has rings on the shoulders, one of which has a combination compass/thermometer/whistle attached to it. The other one is to clip my camera up out of the way when necessary. I use small carabiner clips for this.
I don't wear it for quick jaunts around the neighborhood, but for day-long birding excursions I do, and usually don't use a purse or backpack. I can put all the essentials in and it's cooler than a full backpack, and easier to get stuff in and out of than a backpack. I don't usually carry field guides into the field anymore, but one pocket is large enough for one if I do. If I'm out for a marathon session, like a Christmas Bird Count, and I am taking full meals, I have a small shoulder bag I use for that. I often bird in very hot areas so cool is important, but the vest will fit over a jacket when needed.