VNMC Tigers were the original pattern, until US involvement it was the only pattern which is why it was the most commonly seen early war pattern, The fabric was widely available in huge bolts across RVN - so much so that it still is available which is why you find copies of VNMC shirts with dodgy patches on them being sold as "original" - Vietnam had no exclusive patterns of uniform except the ARVN never wore US jungle fatigues, however you will find US personnel wore all sorts of RVN camo, even the later ARVN Airborne 'flower power' version of ERDL made its way onto US troops. That is basically the only rule, we even know from Rolando that some line grunts wore Duck Hunter boonies on base (yet everyone will tell you that Duck Hunter was a USMC thing). There are a great many pictures of Seals wearing US Army tigers, and i'm sure if you get the magnifying glass out you'll see Army troops in VNMC tigers. I was told recently that a Cav vet who died recently told the guys in the British Legion that when he was in Nam the only time he ever saw anyone in any tigers was on the REMF's? That comes from a guy who did a tour with the Cav, extended as a doorgunner, rotated did his SF training and volunteered to go back with SOG - It all comes down to who saw what, which comes down to what missions you were on, what CTZ you were in and from there what bases you visited?
The one thing that you don't see is Army grunts carrying the older outdated USMC webbing as that was largely defunct WWII kit and the USMC only get new kit after the army have had it for years