VMCa for a light twin is a certification speed measured under specified conditions. It does not vary with anything. If the conditions vary then the speed you are measuring is no lionger VMCa as defined!
If a pilot wishes to turn towards the live engine because he is struggling to control the aircraft at minimal speed he should request the circuit direction he requires. It is an emergency and it is the ATCOs job to give him what he asks for, questions can be asked later.