actual Vmc can be at a higher airspeed given the higher DA, lower power, higher temp, lower pressure, CG, etc. Vmc can also be at a lower airspeed at a lower DA, very cold temp, etc.
Desert, you've got it backwards. Higher DA, lower power, higher temp, lower pressure will all get Vmc to be lower since Vmc is an indicated (calibrated) speed and all of those things will reduce engine power.
BPF, I agree Vyse could actually be considered the more important value here. However, it's important to be teaching students correctly the first time. There is a lot of misinformation out there about Vmc. Just because it's possibly not as important as Vyse doesn't justify continuing to teach it incorrectly. As an instructor I've come across MANY areas that are incorrectly taught and it's frustrating when you're trying to explain to your senior instructor that they're wrong... most of them will quote things they were told with no research or reference or even any logical reasoning! My instructor who was teaching me for the flight instructor rating told me that I should teach that all forces are balanced while the airplane is in a level turn. He justified it because it's "easier for some students to understand".
Btw, the forces in a turn are NOT balanced, if they were then the airplane wouldn't turn. The correct diagram shows less forces... wouldn't that be easier to understand?! I'm getting off topic a bit but this is one of the main reasons why we have so much misinformation out there!