Hello!
For Vietnam you need the white stars (easy) and the "hood numbers" (serial number) and the "bumper codes". Those are harder to get, because they need to be white, and you have to do some research to get an idea about what they need to say. Usually t's pretty hard to find a matching hood/bumper combination, because IF you get a photo it's usually from from or from side and two or more photos showing the same vehicle in tne same time frame is a real modelling treasure...
This link should give you an idea on what the regulations were - real life is of course something else - you got to do research, like I said...
http://www.vietnam.net.pl/TB746/Section3.htm#Agency%20Identification.
Now for the mods on the truck - this instructions show the duece and half in a Vietnam configuration:
https://www.scalemates.com/products/img/2/4/0/163240-32-instructions.pdf
Low tailpipe and old style lights on the rear mounted under the bed and not on the frame would be two things to look out for.
If you need old style lights - I got you covered:
https://www.shapeways.com/product/549MYYM6B/1-35-us-military-blackout-tailllights-24x?optionId=68738639&li=shops
So it's quite a project but it can get you something nobody else has. I had something similar in mind with the M49 tanker. Good luck with your project - I'd love to see some pictures - and have a nice day
Paweł