Hi Vladi,
I've found a couple of pics of ARVN backpacks for you. It is certainly possible that SAS troopers on patrol in Vietnam would wear a wide variety of webbing, including British, US, Australian and Vietnamese made packs. The Osprey book "Vietnam ANZAC's"has some excellent colour illustrations showing the ARVN backpack and the exotic items of equiptment worn by Aussie SAS.
Hopefully Chrisr will read this and bring his first-hand knowledge of the subject to our attention.