The topic came up at Black Crusade forums, but since vehicles are a larger part of Only War (and it's being written right now), it seems appropriate to discuss them here:
What happens when a Warp Weapon hits a vehicle?
A) The Warp Weapon trait works normally mechanics-wise - the vehicles' AP are ignored and damage is deducted directly from the SI.
B) The WW trait has no effect upon vehicles as they're just too big - calculate damage as if the weapon had no such trait
C) The WW trait means that the weapon affects only living beings - the weapon ignores the vehicles AP, but it doesn't deal damage either unless by accident or design the weapon hits one of the vehicle's occupants within.
D) The WW trait is reduced, but not ignored due to the thickness of the vehicle's armour - WW is considered as a certain amount of Penetration (flat 20? Half the vehicle's AP?) when used against a vehicle.