Waypoints? What are those? ;D
Mkay, to build off of General MacGuba up there:
I belive the game engine works something similar to this. When a player enters a lorry, the lorry is entered into an array. This array means that the lorry is availible for any member of the player's group including himself. When you assign the player to another group, the lorry is still attached to the player, and therefore his group.
unassignVehicle is one way of doing it, but I have had some minor issues with it in the past. All unassignVehicle does is order the unit to disembark. That pointer from the lorry to the group still remains. This leads the game engine to believe that the unit should still be inside the jeep. Instead, try leaveVehicle. This unassigns the unit and any group members from the lorry, therefore there is almost no chance you will be ordered to re-embark.
The game engine stuff could be completely wrong. I'm not genius, and I'm not a programming nerd, so don't quote me on it. I would assume it probably works along those lines. But stick with group/object leaveVehicle lorry. That will do you better.