Are you using a human heating pad with it's built-in temperature control or a UTH specifically made for animal vivs that is being controlled by a thermostat?
I'll admit that I have one viv (my isolation viv) on a human heating pad, but you are correct, they are still too hot on the lowest setting. What you need to do is layer towels between the viv and the heating pad. Depending upon the thickness of the towels, you'll need about 2 of them folded so one is 4 thicknesses each and the other is 3 thicknesses each, but you will have to adjust it as needed for your particular set-up. And be advised that the heating pad will eventually break under that use if used continually. I use mine strictly on a temporary basis only for sick snakes.