Virtual Mass:

A factor that expresses the dynamic efficiency of the bow and arrow combination. It is the weight in grains of an imaginary mass shot from the bow bow at the indentical instant and with the identical velocity as the arrow.

The virtual mass represents all of the inefficiencies that are inherent in the shooting of the bow and arrow combination. The sum of the kinetic energy of the arrow and the imaginary kinetic energy of the virtual mass equals the total stored energy the bow.

The lower the value of the VM, the higher the dynamic efficiency of the bow and arrow combination. The VM concept is not totally correct whan applied to the dynamics of the bow and arrow combination, since it is not a constant for every given bow. It is close to constant for many recurves and longbows, but varies with arrow weight for most compounds. It is extremely useful for interpolating and extrapolating performace (primarily arrow velocity) for any given bow when dealing with arrow weights that have not been specifically tested. It is general practice to list a value for average VM in lieu of providing a full spectrum of this factor for the wide range of arrow weights tested.

That's from Norb himself. So the short version is that VM is the imaginary mass to account for the difference between the theoretical total energy in the bow/arrow combination, and the real world results that show the inefficiencies of that bow/arrow combo in dynamic action.