I tried everything, Tamiya XF-74 looks pretty good.
But here is the issue that people don't talk about that much, Vietnam is a sunny place. All it takes is a few weeks and the color is bleached out. It's also a sandy place so the earth and sand get into everything changing the color.
So if you want to build a Vietnam era model you have some issues to consider.
How long has the vehicle been in country. And under what use, was it a M113 jungle busting or a REMF M151A1. A few weeks in the jungle and the vehicle looks more of a light green with scratches all over the side, M48s will have bent fenders or even not have fenders. M113s will be missing lights and other parts on the glacis plate. So you could imagine what it did to the paint.
How many times has the vehicle been repainted.
If repainted what paint was used. Many times they used what they had and mixed other paints even motor oil in it to make more. So you will see many shades if newly painted.
What year, the paints changed and availability was an issue. I see new vechiles coming over in the early years have the gloss green and later the lighter flat green.
What season, if it was the rainy season the vehicle will be darker because of the humidity and rain. For instance any Hue Tet battle vehicles and figures would be darker at the beginning of the battle and lighter toward the end when the rainy season was over.
So to paint any Vietnam era vehicle or figure you have to consider what year, what time of year and what part of Vietnam it was. Along with those other factors I mentioned.
So how do you find out what color to use? Simple get a lot of pictures and use one to make the color right. Make sure the picture is not color corrected and is original or if color corrected jibs with other pictures.