Take a look at www.sass-kit.fsbusiness.co.uk I had them modify their standard aircrew vest to my specs. Came out great, they also do smocks and carry PSK type stuff.
I guess to start with you'd want the material used to be right for your climate and color or camo to your taste. The number, size, placement and method of closing pockets needs to be worked out. D rings, pen/pencil slots, epaulets and type of collar. Do you want a hood built into the collar? Pockets on the inside ?
I went with fairly heavy matterial and a nylon mesh back, and added pockets, as many as I could fit on. I had a good idea of what i wanted to carry in the vest to start with so it would be balanced when worn. I leave space so I can add additional items if needed, like gloves and a hat. The old "photojournalist's" vest from Banana Republic has worked well when I didn't want to wear camo. Also tried adding pockets to a BDU top.