I'm gonna take a stab and say that virtual mass is the amount of energy inefficiency that is not passed to the arrow, similar to if it was mass weight on the arrow itself.

Using Norbs report above, if you had a bow with 100% efficiency and added 143 grains to the arrow, the resultant reduced speed is comparable to what the bow does as at less than 100% efficiency. It's potential energy that is lost in the transaction of flinging an arrow.

Did that make any sense?