Open the editor, any map.

Place any group, naming one of the units Todd.

Place another unit a good distance away, at least 1km. Name it john.

Place any vehicle of the following classes: "Wheeled_APC_F" or "Tank_F", such as a T-100 or Madrid. Name that vehicle APC.

You can also place a vehicle of any other type, such as a cargo van. Name it Van.

Run the mission. Open the Debug Console, enter the following:

(group todd) addVehicle APC; (group todd) addWaypoint [john, 0];

Press LOCAL. You'll observe the group entering the APC or Tank and then sitting their for eternity.

To get them to move, once they are all in the APC, enter this into the Debug Console:

(units group todd) joinSilent (createGroup east); (group todd) addVehicle APC; (group todd) addWaypoint [john, 0];

Hit LOCAL, and watch things work just fine.

Restart, and this time enter this into the Debug Console and press LOCAL:

(group todd) addVehicle Van; (group todd) addWaypoint [john, 0];

Observe as the vehicle is used correctly.

I've attached a repro mission where you can have a group enter a few different vehicles by using Radio Triggers (Press 0 -> 0, then select which vehicle you want them to enter)