i doubt tat a wing would create tat much drag, a roof basket, or a front rack would probably cause more drag then a wing. just look at big rigs, its more important to push air out of the way then it is to smooth it as it leaves the tail of the car.
as Joe_Black said, just cause they look similar doesnt meant tat they are, im not sure if isuzu acutally wind tunneled the vx, but i would assume tat mitsu/dodge would(seeing how the stealth/3000gt was their flagship car). their purpose might not be the same, i would expect the stealth would try to get the air off the rear truck deck to help minimize drag, just look at where the steatlh's wing is mounted, its right behind the window. it would probably make more sense (might not look better or fit) to use a wing off a hatchback, ie civic hatchback.
but to each its own, i personally would go for it since it will give the vx a more agressibe look.