Vmca
CG
In general, the further aft the location of the CG, the shorter the arm of the vertical stabilizer and rudder from the location of the axis around which the the aircraft yaws. With a shorter arm, any yaw force generated at the vertical stabilizer and rudder will have less effect. Or put another way, fewer lb/ft of torque are applied about the vertical axis of the aircraft. So Vmca will be higher when the CG is at the Aft limit than when it is at the forward limit.
Weight (Mass)
In a twin engined airplane, the total loss of thrust from one engine will induce a given yaw force at Vmca. The yaw rate generated by the asymetric thrust will vary somewhat according to the present mass of the aircraft. The lighter weight aircraft will generally experience a somewhat higher yaw rate when experiencing a loss of thrust from one engine than the heavier aircraft will. A given amount of force applied to an object will cause the object to accelerate at a rate according to it's mass. The lighter the aircraft, the more quickly it will yaw and yaw-induced roll into the failed engine. This can be important since a the aircraft must be controllable within 20 degrees of heading during testing.
Determination of Vmca during certification
As a part of the certification of a multi-engine aircraft, Vmca is determined according to the certification criteria of the regulating authority. This speed is generally determined for the aircraft in the weight and balance condition and configuration which will result in the highest Vmca. Detailed testing criteria is available
HERE for FAA part 25 aircraft.
PDF file
Lighter and a further aft CG will normally result in the highest tested Vmca. I'll leave it for someone else to have a go at Vmcg or to make any corrections or additions to my simplified response to your question.
Best regards,
Westhawk