Technically a Vmc demo as performed during a ME check ride or skill test is not how they determine Vmc.
With a Vmc demo we are established SE with the "dead" engine windmilling.
The gradual yaw will lead to roll etc as FFF describes.
The "real" Vmc definition is where both engines are maximum rated power and the critical engine suddenly fails.
If flying below Vmc the very abrupt yaw will cause enough of a rolling moment to turn it upside down.
A Vmc demo will not.