When you are worried about how cold your chickens are, go you, and wiggle your fingers deep into their feathers. You will see they are toasty down near the skin. :)
I haven't used Vaseline on combs, and I think feet will be fine without it, because when they settle down on the roost, the...
As an Iowan, I say "no" to heat.
I used heat once in my coop, the first winter, that was the ONLY winter I have ever had chickens suffer from frostbite. The heat increases the amount of moisture the air can hold, which increases frostbite risk.
Even when we had a winter a couple of years ago...