Keep in mind that often the blouse and the trousers didn't match. They faded at different rates, the trousers might get ruined and replaced and so not match. There's a wide range of color variance that could reasonably be expected in the field, so there's no magic bullet color, just a basic tone.
Some sets were almost a lime green, while others were more of a traditional OD. You can mix and match to create some contrast. It helps to find color pictures, or better yet, an actual example of the clothing.
I use Humbrol 94 with a touch of black and then lighten it with white. I'll also add some yellow and sometimes blue to pull the color in the right direction. It needs to be generally on the yellow side, but it also has to have a gray quality, and it should also be a light OD green. No clus what Vallejo colors to use, but you can mess around with mixing a tan, black, white and yellow and see what you get.
This is about as exact as I can get with Photoshop in the middle of the night, but here are some color chips that might help get the idea across. They illustrate, roughly, the degree of variance. i think the first one is the closest to the most common color.
Also, here's a site for some kind of reenactment group wearing period uniforms. Aside from the color of the utilities, also take note of the colors in the Camo helmet covers. I can't tell you how many times I've seen a good Vietnam figure ruined by painting the helmet with the modern BDU pattern.
http://www.mooremilitaria.com/photos.htm