I think you can almost certainly make the vest work from your bike battery, but the big difference between your bike and a motorcycle isn't just the voltage. A motorcycle has an engine operated alternator that constantly recharges its battery and provides enough juice to also heat your vest. Without a constant recharge yur bike battery won't last very long with a heated vest hooked up to it.
I got my wife a heated jacket from DeWalt a few years ago. It uses DeWalt 20v batteries and lasts a while. You may want to carry an extra battery depending on how long you ride.
I have the Bosch heated jacket which uses a single 12v battery. Adjustable heat range. It'll get used on those particularly cold New Jersey winter days when I don't mind a bit extra heat along for the ride. It works great but consider a spare 12 volt battery to carry with you if you plan on spending long hours out on the trails or roadways. Stylish, too.
Bosch, DeWalt, Milwaukee, most of the big power tool suppliers have these heated jackets that I guess are a favorite of contractors out on the worksites.
A much better and simpler alternative then tapping into some unknown ebike drive system via backyard engineering.
OK, thank you all for the simpler and better ideas. The auxillary battery to power a heated garment is a much preferred concept as it does not compromise the range/power capacity of the motive battery. My ebike has several auxillary battery locations, so no issues there.