Vmp is not an abreviation I have ever come across in 50 years of helicopter flying. We usually refer to that point as endurance speed as in piston minimum fuel consumption is at minimun manifold pressure so it will be located at the bottom of the power required curve.
You need to get Wagentendonk's Principles of Helicopter Flight from ASA as he explains this well and it is the basis for all the JAA PPL(H) POF exam questions.
