I prepare to stand corrected but I'm pretty sure they are fixed values.
Vmcg and Vmca are more to do with airflow over control surfaces than c of g.
Yes if you heavy it will take longer to reach but once you have the required airflow over the control surfaces you will be able to maintain directional control with just the use of primary controls.
I've not read up Vmca or red line speed on the DA42 is a fixed value of 68kts. I don't recall this value ever changing.
I have not read up so maybe speaking complete tosh!!