Vmc is the minimum speed (in the air) at which you can maintain directional control of the airplane following the critical engine failure, using the rudder and maximum 5° bank toward the live engine.
As far as I remeber, it is calculated considering:
- maximum take-off thrust
- airplane trimmed for take-off
- worst CG position (aft limit)
- maximum take-off weight at MSL
- most critical take-off configuration but undercarriage up!
- out of ground effect
- dead engine NOT feathered unless an auto-feathering device is provided.
- maximum bank allowed 5°
- maximum foot load 150 lbs
Vmc has to be 20% greater than Vs.
Do not confuse with Vmcg (minimum control on the ground) or Vmcl (minimum control during landing), however just Vmc is indicated on your ASI with the red line.
Hope this helps