Vone is correct in that on a medium transport jet aeroplane I drive the Vmcg figures are certainly fixed with reference to calculating manual performance.
However like you I imagine that their must be loading and enviromental variables on them. I imagine that in reality the quoted Vmcg figures are probably worst case scenarios.
I'd imagine that the person interviewing you probably cannot answer such questions to a definite scientifically correct point so don't worry too much. They're basically looking for the fact that you can (a.) discuss the subject knowledgeably and (b.) show the ability to articulate a coherent argument.
Good luck!
