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.

It should affect vehicles as it would affect characters wearing armor, since putting armor on a human being doesn't automatically make the armor sentient and subject to the influence of the warp upon its mind [which it doesn't have.] Also, armor is armor. It works the same on vehicles as it does on people and should be subject to the same rules.

I would change the warp weapon quality outright o not leave us with a hole in rules. Really, there's no reason not to use warp weapons when a warp dagger is FAR MORE EFFECTIVE than a vanquisher cannon could hope to be at the moment.

A Warp Weapon ignores up to 10 points of AP, in addition to its standard penetration if any. If the weapon is Two-Handed or improves the user's Unnatural Strength Characteristic, add 5 points to this effect [cumulative with eachother].

However, any Hexagrammatic Generators or wards on the target negate the Warp Weapon quality entirely

The Vanquisher Cannon isn't that bad…. though it might need an additional 1d10 added on. You have to remember that it has the Accurate quality.

Only in the recent errata did accurate actually do something for the cannon. And even then, a solid hit brings it up only roughly on par with… a normal blow from a lascannon.

But a chaos marine with a warp sword is still probably putting both of them to shame.

I think that has more to do with vehicles and their obscene armor than it does with the power of the Vanquisher.