My Aerostich heated Jacket liner just has an on/off switch, If I find it's getting too warm I can switch it off.
Its warm enough that I've actually just slipped it on without a jacket before and stayed warm.
a suggestion with vests or liners is to make sure its up close to your body ( I hate to say tight, but snug?) that way you can reap the benifits from the electric coils.
Mine is an XL and its actually a little loose but still keeps me at least 10-15 degrees warmer and thats nice on my cold ride in the morning.
I rode a couple hundred miles in 30 degree weather and didn't even notice I had left the vents open on my Darien till a riding buddy pointed it out. I think a perfect setup would be a snug electric vest under a (non electric) fleece liner under the jacket, that way you can shed stuff along the way, and with just the vest, turned off, still have core temps ok and can easily switch it on as the sun goes down without stopping to put the liner back in till you have to
Here in the south its often cold in the AM but nice and sunny by afternoon, sometimes I'll wear the electric liner under my perforated leather jacket, warm in the am and cool for the afternoon ride home