1) Dynamic Vmc : the a/c is with flaps TO and gear UP, dynamic engine cut, not more than 15° heading change without exceeding 180 lbs rudder pedal force. This test is more critical at low weights (less inertia, so more swerve). and AFT CG (less vertical stabilizer restoring moment).Pilot should not apply corrective action for 1 second. In this condition, also the 180 lbs limit is normally not a factor.
2) Static Vmc : same aircraft contitions as 1) but , with one engine cut, the a/c is slowed with max 5° bank toward the good engine, until rudder pedal force reach 180 lbs(or lat. control force reaches its limit, which is seldom the case).Normally in this situation the most critical CG position is FWD.
All those tests must be verified for all the approved take-off flap configurations.
The published Vmc is the higher off all the above speeds.
For jet a/c, the Vmc starts to decrease at the OAT/Alt combination where rated EPR cannot be maintained, but this is not true for turboprops, also if flat rated to whatever altitude, because if it is true that the rated power can be maintained up to, say, 20.000 ft, propeller thrust always decreases with increasing TAS. This means that, for the same CAS , TAS decreases with altitude, so does the thrust. For turboprops the Vmc is so tested at different altitudes and then linearly extrapolated at sea leve/ISA conditions (in order to avoid extremely risky tests at low altitudes
I lived five of my best professional years doing engineering flight test, and I am pretty sure of the above.