I have no way to confirm this, but based on some of the industrial uses I can imagine for vulcanized rubber sheet stock, here's what I think happened. The original vulcanized rubber sheet was die cut into gaskets and washers, etc. Someone, maybe in Africa, started stringing small washers and liked the "look". It would have been very easy to start ordering large quantities of "washers", made to a specific size and perforation, and viola, the vulcanite bead business was born. After hard PVC sheet stock was widely available (maybe after WWII, or later?), it would have been much cheaper to make the exact same items from PVC sheet, and they would have been made by the same die-cut parts businesses. I can imagine that boxes of these disks were shipped to Africa, where they were strung by hand into various designs and color combinations.

The Sick catalog shows only black and dark red vulcanite beads. I'm betting that any other color disks we find today are cut from PVC sheet. I'm also betting that I find a few vulcanite disks on the strand, and I think they are going to be the thicker disks - there is a lot of variation. It's a lot easier and cheaper to make very thin PVC sheet than it is to make very thin vulcanized rubber sheet.