What is the function of Vm (Voltage max) in this closed-loop feedback?
It seems to be something like a voltage regulator. In this case, the purpose of block 1/Vm is to transform the voltage at the output of the controller block Gc (Vc) to a duty-cycle value d (between 0 and 1) to be applied in the Gp plant. This occurs by dividing that voltage by its maximum value Vm.