Adam the point of the Vmc demo is to demonstrate that with wings level, climbing, as you approach Vsse you will reach full rudder deflection. By applying around 3 degrees angle of bank towards the live engine, directional control can be maintained with less than full rudder deflection, at Vsse.
The 2-3 degrees AoB, or around 1/3rd ball side slip, uses the horizontal component of lift on the around 170 square feet of wing to reduce substantially the side slip beta on the tailplane and rudder drag (around 30 square feet), fo a very small reduction in lift and a slight increase in aileron drag. The aircraft is now flying straight with minimal fuselage drag. As lift=weight in steady un accelerated flight, the closer to MAUM, the stronger is the horizontal component of lift, hence being closer to MAUM will result in a lower Vmc which is cpunter intuitive.
It is a principles of flight demonstration and not lets show the student a departure from controlled flight at Vmc.
The classic mis-management scenario of the demo is in a summer environment, high density altitude, the instructor ignoring Vsse, going to an incipient departure of controlled flight, except that Vmc is below Vso (more pronounced due to less power on the live engine because of density altitude, and therefore Vmc well below Vs) and the aircraft departs into an unrecoverable flat spin.
The PA-30 AFM is quite explicit.
