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Ordinance templates that aren't centered over a vehicle but touch it resolve the blast at half strength.

If you are in front of a vehicle, and drop an ordinance template on an infantry model that is behind the vehicle such that the large template also hits the rear of the vehicle...you get a half strength hit.

Do you take it against the front armour or rear armour?

*edit*

Nevermind, found the answer on page 60. The hit is resolved at half strength on the facing the blast is on (rear armour in the case given here).

The centre of the blast marker ends outside
the vehicle, but part of the marker covers its
hull. In this case the shell or missile has missed the
vehicle, and only some shrapnel clanks against the
vehicle’s armour. The armour penetration roll is
resolved against the Armour Value facing the centre
of the marker, regardless of the position of the firer,
and the weapon’s Strength is halved (round down).

Page 60 of th BRB says that you take the facing towards the centre of the marker to represent the shrapnel from the blast. i remember this as it came up today, damned vindicators.

you found it