Whether you get a jacket liner or vest, make sure they fit snugly. If they are loose, they won't transfer heat as well to your body. If you can try one on somewhere, that would be best. The IMS is coming up soon and a few vendors will be there selling stuff (Gerbing's always is) but that is already halfway through winter. I have a jacket liner and bought it mainly for the arm warmth. Heated grips but not gloves (gloves on my wish list). Just like your gloves, you will wonder why you didn't get one before. At a minimum, it will take away some of the stress while riding in the cold for a more relaxed ride.
I commuted I-95 for years into DC with nothing but a thick jacket and ski gloves. I never thought to buy this stuff. Now I wonder what I was thinking back then. I now wear the jacket liner if it is 50d or below. It has a controller so I can determine how much I need. The grip heaters are always there, they're great for that surprise cold temperature.