Tyger,
FFF touched on it and from what I can recall (all my notes etc are happily stored away in a warehouse the other side of the Atlantic !!) Vmca is to do with a loss of control of the a/c. Vmca is based on failure of the 'critical' engine which is related to four factors. Don't quote me but I don't think the Duchess has a critical engine. Critical engine is the ........... ahh sod it, I did a quick butchers, see the following link
http://www.copanational.org/non-memb...etyPPDec00.htm
Enjoy .........
CG.