All factors related to Vmc have to do with rudder authority. Think through all of the various factors in terms of whether rudder authority increases or decreases. Increased rudder authority = a decrease in Vmc.

Any conversation regarding Vmc should start with a conversation of the definition of Vmc. What is Vmc? The speed at which you loose directional control of the aircraft. In other words you loose rudder authority. So you must believe that the rudder is the primary flight control for heading. If you are out of rudder... you have lost directional control. Whenyou loose directional control, then you Vmc.

How dangerous is Vmc? Or perhaps this should be framed as, "What is more dangerous, the stall or the Vmc?" For all the conversation that we have in multiengine training regarding the dangers of Vmc it is the stall that is the most dangerous. You can Vmc the airplane all day long... but don't stall. (Problem is that you are probably going to stall just before you Vmc in say... a Seminole.) Look carefully at the graph that shows stall speed and the curved line of Vmc; move the Vmc line back and forth as you apply the increasing and decreasing factors that affect Vmc.

As an aside, be sure to make a clear distinction in your head between the factors that affect Vmc and the four factors that make the left engine critical.