Manual to be Updated

[Richie61]

Technically, the unit doesn't have vehicles in the sense of physically present vehicles.

It has a foot state and a state for the movement type used by its transport.

It doesn't switch from "Men" to "Vehicles" or from "Guns" to "Vehicles".

In order to regain the ability to switch to its transport state, it has to end and start its turn in a hex with its HQ (an actual HQ unit, not a "stab" or "headquarters" combat unit).

It's me. David looked at the file and set me straight. Thanks to all of you guys for helping me understand it.

From the manual;
 
It is possible for motorized and mechanized
infantry units to be classified as "On Foot". This
means that these units have left behind their
transportation and are advancing on foot. While in
this state, the units move as though they were
normal foot infantry. At the beginning of the player
turn when the units are stacked with their HQ unit,
this state is removed and the units return to normal status.
 
Just tried it and moved the HQ and unit you wanted loaded up into the same hex – voila they are back in vehicles. Note the message that appears at the beginning of the next turn.
 
Why does it work this way? Going on foot means the vehicles are moved away a safe distance in the rear. The moving to where the HQ is represents the delay to get the vehicles back and the unit organized.