Because the speed of the vehicle is extremely small compared to the velocity of the AP round, I think you'd find that the vehicle's velocity will contribute little to the round's Kinetic energy.
Eg if the Velocity of the vehicle is 1/100th that of the AP round, then the contribution the vehicle makes to the AP velocity vector would be +0.001V, and so the KE would have an additional 0.001x0.001 contributed, which is miniscule.
Of course this doesn't consider factors like AP round terminal velocity due to atmospheric resistance etc