Uhm, except for Ebob ones that I have still to see in the metal I think Forceof Arms vietnam range is probably the best one around. Good poses, good detail and accurate equipment. Also it has a varied selection of light and heavy webbing.
Now...
the range is limited. Only US Army and Main Force 9th Division at the time of Junction City.
Then we have TAG and Baker Company (sadly no Piccies with me... I promise for xmas pictures a plenty maybe even a full game!) Tag cerainly are better sculpted than Baker company, but baker company variation is simply awesome and they are the only 28mm manufacturer that has real marines and ARVN (yes M14 marines too). And I like them. My USMC force is Baker, my 1st Cav is TAg and the mechanized boys are FoA. I think the TAG VC are pretty bad. Do not like the weaposn exxageration and the poses are more comic books. Also they do not seems to poprtray the real vietnamese an anatomy. Again they seems stright from a marvel comics... NVA is better.
I have some westwind. uhm... plenty of variation, but the quality is hit or miss, nice peasant pack, there is no difference between Army and USMC and the montagnard pack is not 100% what I want. But I will say Westwind as average is not bad.
Saw Parkfield and I like some but not all of the poses. Some are a bit stiff for my tastes.
Then I have eureka australian and VC. I love the VC. The best I have. Also the Aussies are pretty good. They are a bit small. You cannot mix with TAG at all, but they do not seems out of place with Westwind, FoA and Baker.
I never use the ATF and the local VC from Eureka with other forces anyway.
All in all I think you can freely mix and match. Wth the exception of not putting eureka VC with TAG ones almost anything is more or less compatible.