The cuff of any boot is really only a half circle of solid material with the remainder being the buckle straps. What will happen with the vac process is that the cuff will take a set at the position where you buckle it before the pump is turned on. This effect can be achieved with other boot fitting methods (as can some of the other things that the vac process does). The difference of course being that the vac process does all the steps at once and does provide effective tightening of the lower shell and that really cannot be effectively done by normal means.
If I were trying to reduce volume in the cuff area, I might form the boot with the fwd lean spoiler removed, then replace the spoiler after the process.