There is a lot of speculation as to whether or not the fiberglass stocks were every issued in Viet Nam or anywhere else. I have never heard of one being brought back as capture rifle from Viet Nam and I certainly didn't see any of them in Africa.
Word is they were purpose built for commercial export.
The rifle I have with one is no more accurate than any other SKS whether it's Chinese, Yugo, Bulgarian or Russian.
As for standing up, it's fiberglass. It will be every bit as durable as any other fiberglass stock.