Regarding the suitable type, gauge, and brand of wire to use for electric heated clothing, consider the following:
1. Gerbings and Warm-n-Safe have long since figured this out, so use their products as benchmarks.
2. Google for "electric heated jacket" or similar key words, and you'll find plenty.
3. IBMWR website has archives about how to make electrically heated clothing, with guidelines on design, materials, sources, prices, layout, etc..
4. The US Army Air Corps and others were using electrically heated high altitude flight suits in WWII, so there is nothing new in this idea. These suits frequently appear on eBay, so you could ask the seller about the wiring.
5. I have Gerbings' jacket liner, pants liner, and glove liners, along with one HeatTroller unit. The HeatTroller works well with just the jacket, but adding gloves and (especially) pants liner, and it's not so hot, except around the waist, where it's too hot. There are some areas that are not hot enough, so this tells me my Gerbings could use a tune-up.
6. It would work better to have several different HeatTrollers or other suitable pulse width modulators, to control different areas with different heat settings. The multiple HeatTroller does this and has a warranty, but a PWM from an electronics supply house would be cheaper if you don't need a warranty and your time fiddling & tweaking has no monetary value.
7. A good second-hand Gerbings or Warm-n-Safe set will probably be cost-competitive with whatever you can make yourself, without the hassle.
8. Fuggetabout using a vest (without sleeves or in some cases a collar) if you live in a four-season climate or plan long rides. The 'warm body core keeps hands and arms warm' theory is inoperative for more than short rides in cold weather.
9. Somebody posted that they bought a down jacket liner (with sleeves) at Target for ~$40, and that it packs pretty tightly. TELL ME WHICH TARGET STORE!!! This would make a nice addition to my Gerbings set and works off the bike, too. Great idea.