I'm just taking a quick stab at this…
For every full 10 points of armour on the facing that was hit, reduce the Concussive (X) rating by -1. The Concussive rating may drop to zero or lower, or it may go higher than its original rating score, but this just reflects the modifier to the Toughness/Operate Test to avoid crew members being Stunned/Fatigued.
A Battle Cannon (Concussive ) hits a Leman Russ bang-on from the front (armour 40); Concussive (3) drops to Concussive (-1), meaning the Toughness Test is Ordinary (+10). A hit against the side armour (32) would make the Toguhness Challenging (+/-0). A Hunting Lance (Concussive ), using the front armour example above, would become Concussive (-2), meaning the Toughness Test is Routine (+20).
Using an Astartes Thunder Hammer (Concussive ) on the front of a Leman Russ would still leave the crew making Challenging Toughness Tests to avoid being Stunned.
(RAW) Critical Damage of 1 either Stuns or causes Fatigue, depending on the location suffering the hit. Stunning effects to Weapon, Turret, or Hull only last one (1) Round, while the Fatigue is 1 Level only if the Operator fails a Challenging (+/-0) Operate Test. Let's just apply the Test modifier to the Operate Tests the same as we do Toughness Tests.
For the purposes of applying the Concussive Quality to vehicles, the following four vehicle Traits might be considered:
Open Topped Crew of Open-Topped vehicles may be targeted separate from their vehicle by attackers making use of the Called Shot Action (which may be punishment enough). Unless an Open-Topped vehicle is also Ramshackle, I'd say apply the Concussive Quality as outlined above.
Ramshackle These vehicles are of dodgy construction, so perhaps the Concussive rating is increased by +1 when used against these vehicles.
Reinforced Armour This is like the vehicle version of True Grit. Vehicles with this Trait reduce the Concussive rating by -1.
Super-Heavy Due to their immense size and mass, these vehicles reduce the Concussive rating by -3
So we know what modifiers to apply.
Now, when to apply the Concussive Quality to vehicles?
When striking/shooting a vehicle with a weapon with the Concussive Quality, any to-hit roll equal to or less than the wielder's WS/BS Bonus times the Concussive rating has a chance to Stun the crew (as detailed above); these numbers are easy to find and multiply. WS 4 times Concussive (2) = 8% or less on the to-hit roll. Then, if Righteous Fury is generated, the crew must Test vs Toughness/Operate as appropriate (with Weapon, Turret, and Hull being Stunned for 1 Round, or the Operator suffering 1 Level of Fatigue in the case of the motive system being hit.
An ideal scenario: WS Bonus 7 Gunner shooting an Earthsahaker Cannon (Concussive ), hits a Ramshackle Munitorum truck in the rear (Armour 10), needs a 35% or less on the to-hit roll and needs to generate Righteous Fury, and the Crew still get to make Arduous (-40) Tests to resist being Stunned/Fatigued. Even then, the truck is probably blasted to atoms and the crew/passengers will be lucky to be alive, let alone Stunned or Fatigued…