Several observations ..
is due to the impact the rotating vortex trailing from each propeller makes on the wing and vertical surfaces behind
Probably a tad simplistic.
sudden power cut, maximum bank angle of 5 degrees, etc.
Generally the FT workup involves establishing what generally is referred to as the static Vmc .. bit like the pilot endorsement demonstration activity. Once this is established, checks will be made with sudden failures to make sure that the (you guessed it) dynamic Vmc situation doesn't provide any unpleasant surprises .. if it does, then the numbers will be revised to suit.
Important to keep in mind that real Vmc stuff potentially is VERY dangerous and one is best advised to stay some speed above the real world Vmc on the day.
an engine failure at Vmca will result in a significant difference in performance
True, depending on Type, but not overly relevant to anything. Vmc is concerned with handling rather than performance. Unless and until the pilot gets the aeroplane back up to a speed appropriate for OEI climb things generally are going to be untidy so far as climb is concerned ..