Density related
I think the point you want to see here is DENSITY.
The higher the density (Lower airports) the engine provide more thrust/power.
If you loose an engine, the operational one will provide more thrust/power and you will need a higher yaw force to counteract this effect, therefore you will need a higher VMCG/VMC speed to help this yawing force at higher density airport or lower altitude airports.
In this case (high altitude airport), your operational engine will provide you with less power/thrust that in the similar situation at a lower altitude airport, therefore your VMCG will be lower.
Viele Gruesse and viel Glueck!
Gus.-