It means Vietnam have imposed capital controls to prevent the sale or purchase of the Dong outside of Vietnam. This allows them to effectively manage their exchange rate.
We visited Vietnam a few years ago and had the same problem but think it was only a relatively small amount we were stuck with. Someone later suggested sending unwanted dong to the Christina Noble Foundation in Vietnam - can't remember if we did or not but am sure they could put it to some use over there.
I gave my few dong away to a few students I overheard discussing an upcoming trip to vietnam in a cafe. About all you can do with it. Great country though. Poor people but with a sense of possibility and promise, you really get the feeling they will do well, unlke other countries you can visit that seem to be basket cases.