Putting together a passable US Army or USMC Vietnam-era impression is pretty easy and not that expensive:
UNIFORMS:
Except for very early (pre-1966) or very late (after 1970), typical US and USMC personnel "in the field" and/or out on patrol would have worn either Type 2 (exposed buttons) or Type 3 (covered buttons) Olive Drab Jungle Fatigues. These were simple cotton poplin olive green versions of modern bdus, with a couple of notable exceptions, such as slanted chest pockets on the jacket and an extra piece of fabric (gas flap) inside the jacket opening.
These modernesque and functional battlefield uniforms replaced decades of hodge podge less-utilitarian wear, and were a huge step forward, being rugged, comfortable, and effective. They were so liked that the pants were often stolen and worn by the Vietcong.
Despite chemical vat dying, the cotton fabric faded in the severe tropical Indochina sun, resulting in color variations ranging from dark olive to light medium green.
1a) An exact reproduction of the Type 3 jungle fatigues ($50/set):
http://fullmetaljacket.com/catalog/repr ... 3f576e8101
http://fullmetaljacket.com/catalog/repr ... 3f576e8101
Another source:
http://www.workinggear.com/gc/customer/ ... ctid=18777
http://www.workinggear.com/gc/catalog/R ... at708.html
1b) Another inexpensive alternative, a 'modern' Tru-Spec solid OD green cotton ripstop bdu set ($50/set):
http://www.georgia-outfitters.com/od.shtml
The Tru-spec OD bdus are the closest in color to OD#107, the as-issued coloration. These same duds will prove useful in many other operations, including WW2 and even modern-themed events.
Avoid the Propper brand OD bdus, they're visibly the wrong color and look odd (too bluish green).
1c) Another alternative, especially for USMC impressions, is an old surplus nylon flak jacket ($?) and simple OD t-shirt ($7):
The old lumpy nylon OD flak jackets used to be nearly free and readily available (back in the 1980's). Today, I have no idea where to get one for a reasonable price.
While not allowed at OP: Hac Bao, just the plain OD green T-shirt was often worn by our troops, when they could get away with it.
http://www.workinggear.com/gc/catalog/R ... at628.html
http://www.militarycraft.com/bdu_camouf ... olycotton/
HEADGEAR:
Period headgear is required at all our historical events.

2a) Actual US M1 steel pot helmets are still cheap, check any surplus store. They changed only slightly from 1942 to the mid 1980's (when they were replaced by kevlar models). Dutch, Danish, Belgian, Austrian, French, early Israeli, and Greek pot helmets were also identical for decades. Any will work. It has 3 key parts, the steel shell, the plastic liner, and the suspension (attached to the liner). The shell goes over the liner, which goes on your head. Also very useful for WW2-themed events ($25):
http://www.sportsmansguide.com/net/cb/u ... x?a=571896
Surplus stores carry these for about the same price.
2b) Cheap plastic version of US pot helmet ($16):
http://www.cushmanpaintball.com/GIHelmetPage.html
Lighter, but crappier than the real mccoy.
2c) If you just can't be bothered with a helmet, get a triangular piece of OD green cloth, such as a surplus bandage, and wear it as a bandana (or "drive-on rag") for that "in country" look ($3):
http://www.galaxyarmynavy.com/item-4348od.asp
2d) As a general rule, no boonie hats. Only special forces, LRRP's and the like wore boonies. That ain't you, son. Sorry to have to break it to ya'.
Some events may allow these, in either OD green or Tiger stripe. Small brim "Gilligan" style was what was used/issued, not the big wide brimmed panama-style boonies you see today.
If you go with a helmet (2a or 2b), as you should, try to cover it with one of the following:
Mitchell Pattern helmet cover, very common in Vietnam, but very hard to find today in decent condition ($15-$40):
http://www.mooremilitaria.com/Cover%20used.JPG
Ebay is your best source for these. These faded badly, even at the time, so even a bad one is okay. Secure it with a piece of black rubber (inner tube rubber works great) around the helmet, just above the brim.

Helmet netting, less common in Vietnam ($7):
-Broken Link-
http://www.ima-usa.com/product_info.php/products_id/510
For USMC personnel, WW2-era "Duck Hunter" camo covers were also still in sporadic use ($30):
http://www.mooremilitaria.com/USMC%20Helmet%20Cover.jpg
FOOTWEAR:
For Vietnam, the US military issued a new boot, the "jungle boot". These mixed black leather and OD green fabric boots were cooler and more comfortable in the tropics than the all leather boots they replaced, and had better functionality (better traction, etc). The heavily lugged "Panama" sole was used at the time.
3a) Exact reproduction Vietnam-era leather and nylon green jungle boots ($70). Not exactly cheap, but you get what you pay for:
http://www.vtarmynavy.com/green-jungle-boot.htm
3b) Cheap chinese knock-off jungle boots ($18-$20):
http://www.majorsurplus.com/Jungle-Boots-P13724.aspx
http://www.amazon.com/Brigades-Action-G ... _pr_sims_t
Look great, but your feet may hate you. Get the olive ones, the black ones didn't exist yet.
3c) And of course, there's whatever kind of boots you already own (quite acceptable for our purposes). Non-combat troops in Vietnam (armor and air crews, for instance) wore black leather boots, so these are decent choices.
Pants legs were typically bloused into the boot tops, to keep the massive leeches and ants found in Vietnam out of one's pants.
To dress up your uniform, don't forget the ubiquitous OD green towel, used to wipe the sweat out of your eyes. This was, according to many, the single most valuable piece of gear one could score ($10 for 2). Or check your local Chinamart or Target store.
All that remains is field gear.
For airsofting, very little of this is actually needed, or even desirable, as was the case with Vietnam vets.... You don't want to hump too much gear through the jungle.

LOADBEARING GEAR:
US Vietnam era loadbearing gear consisted of a canvas (M56) or nylon (M67) wide OD pistol belt, H-yoke suspenders (later Y-suspenders), two rectangular ammo pouches, a first aid/compass pouch, and one or two plastic canteens with canteen covers. A OD buttpack typically completed the set.
Nylon gear slowly replaced canvas gear, as the canvas rotted badly in the tropics.
USMC personnel wore different suspenders, but otherwise gear was very similar.
4a) You can get genuine M-56 and/or M-67 pistol belts, suspenders, and first aid pouches here, but expect to pay a premium for them:
http://fullmetaljacket.com/catalog/viet ... 3f576e8101
http://www.mooremilitaria.com/prod01.htm
http://www.bayonetinc.com/store/categor ... U.S.-Gear/
For those seeking to go all out, these guys on Ebay offer complete sets of M-56 gear, for under $100.
http://search.ebay.com/_W0QQfgtpZ1QQfrp ... 2dventures (Edited by Lurch: The Gear Guys are now selling their LBEs, rucks, uniforms, & other items under the new moniker of Joint Military Ventures, although they're still selling props out of the The Gear Guys eBay store still)
4b) You can get new repro butt packs, H and Y yoke suspenders, and pistol belts here for a bit less, with a bit less fidelity as well:
http://www.fatiguesarmynavy.com/store/c ... Packs_Gear
4c) The cheapest and cheesiest alternative is surplus "ALICE" (LC-2) gear, which is quite similar to the M67 equipment ($20):
http://shop.sportsmansguide.com/net/cb/cb.aspx?a=361084
Comes with belt, suspenders, ammo pouches, one canteen and carrier, and first aid pouch.
To make ALICE pouches look more period, cut the front grenade loops off the mag pouches with a seam ripper so the sides pouches are flat.
4d) One very common item used by Vietnam-era soldiers, but that no one else seems to emphasize, is the cloth ammo bandoleer or "repack kit", used to hold spare M14 or M16 rifle ammo on stripper clips ($6-$8):
http://www.mooremilitaria.com/prod03.htm
http://www.fredsm14stocks.com/catalog/acc.asp
A bit of hunting around can find you better bargains, now that you know what you are looking for.
Keep in mind that there should be no Velcro on your gear and no modern accessories such as drop leg platforms, dump pouches, etc.
Modern tactical gear and loadbearing vests are wildly inappropriate.
CANTEEN:
US canteens started the war as metal, but were later made from injection molded plastic (identical to modern US canteens). Either way, an OD cover/carrier held them on the pistol belt.
5a) US metal canteen with Vietnam-era OD nylon cover ($10):
http://www.armynavydeals.com/asp/produc ... BG217&ST=2
5b) Walmart carries a cheap metal canteen with OD cover, somewhat like this ($5):
http://www.rothco.com/showpics/index.cfm/item/1283
Brand is either "Rothco" or "Ozark Trail", and comes with a canvas web belt. Stencil "US" on the carrier and belt (if you use it) in black.
5c) Another metal canteen, OD canvas cover, and canvas web belt set for a mere ($9):
http://www.basspro.com/servlet/catalog. ... rchResults
Again with the stenciling.
5d) Modern US plastic canteen with modern US nylon cover. You probably have one already ($0).