How can I figure out critical altitude?
Vmc is not the speed in which you can maintain altitude, it’s the minimum speed in which you can still maintain directional control with an engine inoperative in a twin.  Vmc is a design specification and not a maneuvering speed which is established by the manufacturer according to the design requirement of the aircraft.  The actual minimum control speed for a given situation, Vmca, varies based upon a number of factors, including density altitude, loadout, aircraft configuration, etc.  Vmca can be determined by doing a Vmc demonstration, which is a required maneuver to be demonstrated for an airman certificate operating multi engine airplanes.

Critical altitude is the altitude at which above this a turbocharger or supercharger can no longer maintain the maximum design operating manifold pressure for a reciprocating engine.  It is an important factor if you need to know the maximum power output of your engine at any given altitude for climb requirements, ability to attain and maintain an altitude to clear terrain and obstacles, etc., no different than knowing the available power for an normally aspirated engine.