Line of sight for shooting is traced directly from the weapon mountings on the vehicle. Weapons mounted on vehicles often have a limited arc of fire and they may not fire on units that are outside their arc. Trace the actual line of fire from the weapon mounting on the vehicle to see if it will be blocked by terrain or other vehicles.
Diagram: Vehicle Line of Sight. The Predator tank can trace a line of sight to the Vyper with its turret-mounted gun and the weapon in its left sponson. The weapon in the right sponson, however, is blocked by the intervening jungle and cannot shoot.