Vmca is a certification speed; it does not vary with anything; if you change any of the the parameters it is no longer Vmca. As one of the parameters is that its determined at sea level, it is almost impossible to acheive in the air. As with all such speeds you may find differences withinin the definitions of the Certification data. BCAR Section K was the UK certification criteria for light twins prior to JAR23. Essentially it is determined for the take off condition with maximum power on the critical engine, the failed engine windmilling; max AUW and max aft C of G.
Vmc on the other hand is the minimum control speed at what ever conditions its measured under and does vary. It may even be the pilot's personal Vmc when they can no longer maintain straight flight to within 15 degrees.
You can determine a speed at which the aircraft can just maintain directional control however; if the engine were to suddenly fail at that speed, the rate of yaw and roll might well exceed the rate at which recovery action can take place added to which one must consider the delay in reacting.
It should always be rememberd that closing both throttles and lowering the nose immediately solves the asymmetric problem.