The shoulder straps on my 1.5mm and 1mm regular ABS suits can be folded in half easily without cracking. They are very flexible compared to the 2mm acrylic capped ABS and maybe the ABS that other makers use too, so I can understand why you think that way about the priginal ROTJ straps.
If you look closely and freeze frame certain scenes, you can see the velcro on the chest and back plate showing under the straps that are not placed correctly. The shoulder straps are simply vac formed ABS with velcro glued underneath.
The ones in the exhitions could be original ROTJ screen used or may have been made after filming from the same moulds, but one things for sure, the shoulder straps have been replaced. The ABS straps may have been lost or damaged and new ones made from the vinyl ribbing. The velcro is still there on the chest. If the straps were the original ones and meant to be riveted, why would there be velcro?
The vinyl (or whatever it is) is like man made leather. Fabric (like cloth) is a weaved material like cotton, nylon, silk, not like these shoulder straps in question.