There are a lot of sellers out there who seem to be categorizing uniforms and field gear as Vietnam era, but the stuff really isn't.  I do not know if it is a lack of knowledge on the seller's part or a deliberate attempt to capitalize on a growing collectors market.  Whatever the case here are some basic things to know to keep you from getting burned....

Do a little research before bidding.  there are plenty of sites out there with info about the gear, just because the seller says it is so, does not make it accurate or true.  When in doubt, check it out.

If it is made out of nylon, be careful, look before you leap, not all nylon gear was issued in the Vietnam theater of operations, a lot of it wasn't even around. 

Magazine pouches - if it has a plastic closure clip, look it over - if it has the big floppy looking pockets on the side to cradle the grenade, it wasn't in Vietnam.  Easiest way to tell though, if there are no support straps that go from the pouch up to hook on the shoulder straps, then it is the wrong era.  The post Vietnam ammo pouches have eyelets to hook to the shoulder straps, very much like those on the fanny pack, but no straps.

Web belts - if it has a plastic closure buckle - it wasn't in Vietnam.  Two types were used Pistol and Davis.  Pistol belts buckle just like the WW2 belts, the Davis belt has a flat metal tab that inserts into a slot on the other end of the belt and locks in a notch.  Troops hated them, but they were there.

Tri-fold shovels were there, very late in the war, but the green rubber pouches for them were not.  Covers were canvas or nylon

Helmet covers - are they reversable?  If not and the pattern does not look a lot like leaves and such, but more like the 1980 camoflauge uniform pattern then they are not Vietnam issue

Helmet bands -the little elastic bands did not have reflective tape on them.  IT COULD GET YOU KILLED!  Beware of the Marine corp home made rubber inner tube type bands.  They are very easy to counterfeit.

Steel helmets - NO KEVLAR IN VIETNAM! - if the seam joining the rim is in back it was Vietnam, if in front it is WW2.  Helmet liners with the rivet snaps for a leather secondary chin strap are WW2, not Vietnam.

Web gear - if it is canvas and identified as M1956, it was in theater.  If it is M1967 it is mostly nylon and was in theater (watch out for those magazine pouches!)  There was a lot of WW2 webbing that made it to Vietnam, especially in the earlier years.  The mountain packs, identified by the pack frame that resembles the lightweight tropical frame with the leather closure straps on the pouches were in theater a lot.

Duffle bags - if they have those handy shoulder straps on them that allows you to carry it like a back pack, they never sawVietnam, they did not come out until well after the war.

Alice packs and frames - many arguments about this, I say no, based on research.  But there are always the friends who have buddies who claim.......  IF you are serious about collecting Vietnam equipment, either to preserve or as a living history project, avoid them as being controversial.