Yes, Vmc IS based upon the rated thrust, but what of aircraft which operate using several thrust ratings, e.g. TO, TO-1, TO-2.
For the TO, TO-1, TO-2 example given, there are 3 'official' Vmca/Vmcg figures, and there's no harm in advancing to the rated thrust of the day, e.g. TO-2, but
NOT to full TO (Full Rated Thrust). Similar possibilities exist with
Mutt's Jungle Jet on an APR OFF Takeoff, and the operating engine advanced to APR following engine failure.
For the aircraft that I fly (B777) TO-2 is a 20% De-Rate on full thrust, and if thrust is advanced to Full TO following engine failure, you're gonna start going sideways!!
Regards,
Old Smokey