According to the very cheesy Jeppesen Multi engine video Vmca decreases with altitude because the operating engine produces less power at higher altitudes. This is likely assuming a non turbo engine because some turbo engines are rated higher at altitude because the air charge is cooler but they can maintain the same manifold pressure (thanks turbo!!)
My logic is telling me that Vmca could increase with altitude on a turbo charged aircraft for this reason but please correct me if I am wrong