Here are some examples I’ve put together of rucksack setups for Vietnam. These are not copies of specific units or people’s gear just something to dress it up and make the set up look more authentic.
Pretty basic, a canteen on each side, the poncho rolled up underneath, a machete on top, a 5 quart collapsible canteen tied over the top of that, and a claymore bag just draped over it all. The 5qt canteen is a late war item so leave it off it you are trying to show anything pre 1969.
Lightweight Rucksack “Normal Position”
Just sort of combined some ideas from the 173rd and other units I saw in the earlier years of the war. Canteens on 3 of the 4 attachment points and an M1951 Entrenching tool upside-down on the lower left attachment point. A poncho is rolled up and held in the cargo straps with a machete hung from the top strap hanger area. As an alternate I added an XM-28 gasmask bag and stuck the machete sideways over the poncho. Although the XM-28 was a late war item.
Lightweight Rucksack “High Position”
This would be for later war impressions. I have canteens on 3 of the 4 attachment points but have the E-Tool just slid through the tunnel behind the one pocket without its pouch/carrier. I saw that method being used used in a number of late war photos. I have the machete across the top and a claymore bag looped around the sheath. Underneath is a bed roll of a poncho, liner and inflatable mattress. The webewebbiers.com site has some good gear anecdotes and photos from veterans if you want to see some of my inspirations for this set up.
So some closing notes. If you are going for a regular Army/Marine look the later on in the war the more stuff you will want to pile on. But if you are trying to go for a more recon or special forces type look then keep the pack on the cleaner side. They still carried a lot of gear but it tends to be much more tidy. Keep it internal or tied down, as few snag hazards as possible.
I think my favorite is going to be a toss up between the ARVN and the high mounted Lightweight. I suppose if I was going out longer or needed to carry a heavier load then I would use the Lightweight but if not I would lean towards the ARVN. I like how high up on the back it sits, allowing you to use the back of your web belt for gear. Just remember that just because we like or prefer a certain set up that doesn’t mean it’s appropriate for a period. We have the benefit of decades of hindsight and access to gear that men at the time did not have. So make sure to look at period photos to avoid anachronisms.