PER is essentially PVC which has been plasticized and stabilized with acetyl tributyl citrate, instead of the host of phthalate-based plasticizers such as bis-diethylhexyl phthalate (DEHP) whose endocrine-disrupting properties give PVC its well-deserved bad reputation.
Citrate salts tend to be readily water soluble, and therefore this material is touted to be more biodegradeable than phthalate-plasticized PVC. I certainly cannot find any information to refute that claim. ATBC will further degrade into other acetates and butyrates, most of which are relatively common and harmless in the environment (though butyl compounds are infamously stinky; butyric acid is a favorite less-lethal offensive-odor weapon of law enforcement and groups like the Sea Shepherds). All PVC will eventually degrade as the plasticizer leaches out over time, unlike other types of plastics that are so ridiculously inert they'll likely be the fossils of our modern era, but again, the phthalates just aren't a great idea to have in the environment.
Of course, neither is the vinyl chloride monomer. What many proponents of PER fail to mention is that the monomer is worse for your health than the plasticizers. It's carcinogenic in relatively small doses, for one thing. It's also toxic to the liver in even smaller doses, measured in the low parts per million of body mass for an acute exposure. Lastly, the monomer is extremely flammable, though this is mainly an industrial hazard; the polymer is actually among the more flame-retardant commercial plastics, due to its chlorine content (in other words it's already highly oxidized, as opposed to something like polyethylene). None of this changes when the plasticizer does.
In short, while PER is a step forward, there are still plenty of inherent hazards in the production, long-term use and disposal of PER products. PER is also a relative specialty product, whose primary use is a foam product marketed for children's toys and yoga mats. It's not a silver bullet to replace PVC wholesale.