So when fired at a vehicle, resolve its effects as normal. Meaning roll S4 against the armour value of the vehicle. Once you do that, roll another dice and apply the haywire effects. Essentially if you are facing the rear armour of a vehicle or the side armour of a chimera, you have a chance to glance the vehicle from the normal effects of a S4 weapon. Then you roll again and apply the effects of the haywire. Meaning that it is possible to get two damage rolls against a vehicle with a single shot. A glancing hit with a S4 + a roll of a 6 on the armour penetration roll, and then one with the haywire effect on a 2+. Not likely but possible.
I'm sure most of you already knew this, however, sometimes I am a little slow.