I fully agree, could be a long operation to get a vehicle sent from Darwin to Vietnam, you could be there quicker by driving from Singapore or KL.
And just in case you get rejected at the border, when its a land border, you can just go back, but if its a port, you are looking at shipping it elsewhere then, at your own cost.
I dont know, if they will accept foreign cars or not, but I do know that with bikes it is a big problem. Not totally un-doable like Burma I guess, and some seem to have made it (probably just got very lucky I think!) but cant count on succeeding. Before you ship it there, make sure your vehicle will be allowed entry into Vietnam.