Well, Kevin Lyles' book, Vietnam US Uniforms, is a pretty good start. It's all color, shows front and back of real-life people wearing the gear; and the gear is assembled prototypically. It's not perfect; few enough reference titles ever are <sigh>
Shelby Stanton's US Army Uniforms of the Viet Nam War is also a good reference. It's mostly b&w, but the photographs are very extensive. The refence photos are also mostly "in country," too. That's good for being "way it was done," as opposed to "way regs said to." With the caveat, that, sometimes, it was the "odd duck" that was photographed--the exception rather than the rule. But, that's a grain of salt we all have to cope with.
It can be helpful to find some of the FM & TM for the gear (and frustrating <g>). FM 21-15 Care of Individual Clothing & Equipment can be handy.
You can also look here: http://www.hardscrabblefarm.com/vn/index.html
And this, for M-1956 gear: http://www.hardscrabblefarm.com/vn/m56-gear.html
See if that doesn't muddy the water a bit<g>