Vmc is a factor of two forces, both of which act around the CG. You can best visualize and calculate the forces looking down at the aircraft.
First, the engines at 100% power (sea-level is the worst case) produce a specified thrust. That thrust works through the arm over to the CG. We locate the center of thrust slightly away from the center if the propeller to account for P-factor. This is why for Lyco's and Continental installations that are not counter-rotating, the left engine is critical. The thrust force acting at the end of the arm that originates at the CG creates a torque force around the CG. We can calculate this number.
Second, the vertical stab and rudder create a lift force in the horizontal plane. At full rudder deflection we can calculate this lift force vs airspeed. Its a wing turned sideways so you can figure out its coefficients of lift, drag, etc. You can spend a few hours with a program like XFLR5, OpenFOAM, etc figuring out lift vs airspeed graph for the airfoil shaped by the vertical stab and rudder at full deflection.
The vertical surface generates lift force that works around the CG just like the engine thrust does. So you can convert the graph of horizontal lift vs speed to a graph of the torque provided by the force of the vertical surface acting on the arm over to the CG.
Once you have this info, you just take the torque around the CG that you calculated the engine creates, and move across the graph you just plotted for the vertical stab/rudder until you find the same value. From that point go to the airspeed axis of that graph and you have your calculated Vmc.
Flight test will confirm your calculation.
The above is the Cliff Notes version of the calculation. There are more steps that add to the accuracy/correctness of the calculated result.
Best of luck,
Wes