VND isn't particularly easy to exchange outside of Vietnam which most likely accounts for the awful FX you've been given. You'd have been better trying to find someone who is going there soon or living there but here at the moment and exchanging directly with them. You live and learn.
you said border runs by land are limited to 2 entries per year and that he's used them. If he has a tourist visa then there's no limit on the number of land entries in a year.