Vmc and Vmca decrease with altitude when the thrust from the live engine decreases with altitude - thrust is the cause of all the problems. If you have a superdooperturbocharger thingy then thrust may remain constant up to significant altitudes.
I suspect people here are confusing Vmca with Vmc. Vmc varies with asymm power, flaps, drag, cg position, leg strength - and probably something else that I've forgotten.
... Bottom line is to close the live throttle and lower the nose whenever you reach a control limit or get within 5kts of the stall.
HFD