Electric vest powered from 12v battery. Make it yourself for $10. Takes a lot less energy if you put the heat right into the body not air. 20-30 watt should be plenty, but if your going to do it, wire 2 circuits so you can have 3 heat settings.
Made one about 10 years ago for winter motorcycle commute.
http://www.yamahafz1oa.com/forum/sho...d.php?p=922173
Using a battery you'll only get 12 volts, so have to adjust calculation for that.