The short end of it is "everything has shields, even if it doesn't have defense". One could simply just call it "structural integrity" for vehicles that do not actually have defense.
The mechanical change would be that if the "shieds" or "structural integrity" is compromised, armor drops to 0. A certain level of damage would still need to be applied (10 damage per point of hull trauma for personal scale weaponry) to actually damage the vehicle.
This makes a vehicle with damaged "shields" susceptible to damage from ground troops and larger vehicles more susceptible to smaller vehicles. Makes shields a much more important factor into combat as well.