Without really raiding my notes I seem to remember that Vmca is quite specific to each aircraft and that it is established under a particular set of circumstances.
Is it defined as the speed at which:
1. At sea level.
2. Flaps in the take off position.
3. Gear down.
4. Max continuous thrust set.
5. Critical engine propellor windmilling.
6. C of G in the aft position.
7. Aircraft at max all up weight.
Control of the aircraft cannot be maintained with full opposite rudder deflection.
It is a very specific speed which is established under the worst go round conditions. If demonstrated at safe height the excercise provides a useful illustration to the student that there is a speed below which he cannot control the airrcaft in the event of assymetric flight.
I've probably forgotten something here but that's the gist of it is from what I remember back in the old days of twin training. It's a useful excercise which illustrates what can happen with slack attitude and speed control in such situations as engine failure after take off or single engined go round.
