If the unit is already in a vehicle, nothing will happen. You should use moveOut to move the unit out of his vehicle before trying to move him into a new (or same) vehicle.
canReassign: Boolean - (Optional, default true) try to move unit into the first free vehicle cargo seat if cargoIndex is already occupied (since arma3 2.08)
If you place a soldier in a vehicle with the moveInCargo command, he wont "know" he's in the vehicle, and thus he won't disembark properly when the vehicle reaches a Transport Unload waypoint.
Therefore you have to use the assignAsCargo command, in order for the AI to catch on. Something like this:
In OFP v1.96, the moveIn commands will not trigger an associated getIn event. To ensure the getIn event is fired, use the "getIn Cargo" action command.
Killzone_Kid
In Arma 3 when using the alternative syntax of moveInCargo, it is necessary to call assignAsCargoIndex because it is not called automatically.