Have a look at http://s6.zetaboards.com/vietnamairsoft/index/ . Not terribly active, but will give an idea of what's going on in the Vietnam Milsim world.
Have to say, it's good to turn up in a sea of multicam with a loadout thats just that bit different, and makes you stand out in the safe zone. Had a scenario last time I was out where there were people taking pictures of me in my 1st Air Cav gear with a guy wearing 101st Airborne kit. For me, I watched loads of Vietnam movies when I was a kid, and really like the idea of re-enacting it.
If you,decide its not for you, have a look on eBay - original Vietnam kit makes pretty good money. Alternatively I'll give you a fiver for the lot😉