2 points for consideration.

[Victim]

You might want to consider a Wind Wall effect for protecting a vehicle from arrows. It's a much more reliable defense against arrows than the Protection spell since it can't be overwhelmed by damage or bypassed by magic weapons and can protect multiple people. The spell does have its limits though.

 

[moritheil]

Even then, it would only provide the vehicle/creature with Damage Reduction, which would be handy if anyone tried to destroy the vehicle, but would provide no protection to anyone riding on top. And, in that case, you're better off leaving the vehicle as an object, because then it's got hardness- DR that applies to energy attacks too.

I'm assuming it's not an open-topped vehicle, and thus the walls need to be breached to get at the occupants. The OP didn't state.