Hi Mightyman,
This is used as a fire barrier so the mattress can pass the fire regulations. Viscose/Rayon materials are derived from various types of plant fibers (such as bamboo, beech trees, eucalyptus trees and others) and are a “semi synthetic” fabric (see here). I would consider them to be a “safe” fire barrier (see this page and post #2 here).
I’m looking forward to finding out what you end up deciding and of course any other questions or comments you may have along the way.
Phoenix