Take a rectanglular piece of fabric.Measure the space between your arms on your back and make two slices in the fabric. That is it. Making the slices off center will make your vest assymetrical.
With an eye on winter I decided to make this vest in leather so it can be used as outerwear. (But am now also planning on making a silk version!)I chose to leave the leather in it's original shape. It wasn't symmetric but I liked how it turned out.