Just as an aside, I've never clamped a frame while removing headset cups. I just hold the head tube in one hand and bang on the remover with a big hammer in the other hand. The remover is the old trusty piece of electrical conduit, slotted, and spread out a bit to form 4 tabs.
The opposing force comes from the inertia of the frame. I think it's the most gentle way to do it, without risking breaking the frame joints, bending the tubes, or gouging the frame in the jaws of the vise. Also, I wouldn't discount the use of ice, though I've never tried it myself.