I don't think the rudder decreases in efficency with altitude PROVIDED YOU ARE AT THE SAME INDICATED AIRSPEED.
IF I WERE YOU, I would find a flight manual for a turbocharged plane and see the graphs for Vmc(a) and see for yourself what it does, as a turbocharged engine shouldn't lose power with altitude.
For example if you are learning to fly in a normally aspirated Piper Seminole, compare the graphs for a Turbocharged Seminole.
Ihope you will post what you learn.
I have to admit, I was a CFIIMEI back in pre****oric times and I fly jets now, (737), and haven't really thought about Vmca for awhile. I stay away from it!!!!
Also, one other thing...no matter what the book says the speed is, when you start to lose control, you are at Vmca, so get the nose down and reduce power on the operating engine...don't even get close to VMCA at low altitude as this part of multi engine training is one of the most dangerous . One engine go arounds in light twins are very dangerous too...so BE CAREFUL