Based on my reading, "veh_type_code" is often referred to as a "Discriminator", but I haven't seen people make it part of the key.
Furthermore, having the discriminator in the subtypes is redundant and serve no purpose linking the supertype and subtype as they are already linked via the PK.
Since reading more about super/subtypes it seems like the link above is bad design.
If youw ant to see a HUGE rant on this topic, see this thread...