If you are looking to buy items of US clothing or equipment from the Vietnam War era, then you will need to do a bit of research first.
A great quick reference book by Kevin Lyles, US Uniforms in Colour Photographs. It features a lot of genuine items from a few private collections that are displayed on human models for a more realistic view, front and back, of how the items would have been worn or carried, etc.
For a much more in depth guide (1,200 pages) to the uniforms and equipment used by US troops in South Vietnam, you should go to VietnamGear.com Their information comes directly from US Govt. paperwork stored in the National Archives and other such Govt. sources.
Finally, be very careful!