A very good point Cavorting'
My understanding is also that Vmca decreases with altitude. On my twin oral test the examiner asked if I'd flow at Vmca and at what altitude. When I said yes and quoted the altitude he seemed a little concerned. Thinking that I was about to get my instructor in the do-do I revised the altitude up a little. At this point the examiner became even more concerned and went on the explain that as altitude is increased the Vmca comes closer to the stall speed, which is not recommended! I have no dought that after my test the examiner had a quite work with my instructor
A catch 22 situation if there ever was one!