Maybe change S2M to something like Combustion,
Change its duration to 30sec, nerf the increased insanity generation to 100% (should be enough to sustain 30sec more then usual) and keep the 10min cd.
But every for every stack if VF you get, you reduce the remaining cooldown of S2M by 2sec.
Wouldnt make S2M perma active during the last 2mins, wouldnt be able to reach the MH cap easily, and would still be encourage to stay as long in VF as possible - but not only during the last, but you want to really maximize every single one for the CDreduction.
Would also scale pretty good with gear, since with better gear you get longer VForms, and that results in less CD and in stronger 30sec because of the exponential damageincrease until 100stacks.
Numbers could ofcourse be changed