Been there I think in 2005, very rare I got some coins in change but I remember well having a super large stash of bankotes which actually had not much value.
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There are 26,000 Dong in a Euro, so the highest denomination coin (5,000 Dong) is worth less than €0.20. Still that coin should have decent purchasing power in a country like that. Perhaps people just prefer banknotes, or coin supply is small as the old ones have slowly disappeared and no new ones were made.
So the smallest denomination is VND 10.000, which is less than €0,40. It sounds a bit high if they really don't use coins. There are smaller banknotes still in legal tender, but those are printed 25 years ago so they are surely gone.
I travelled through Vietnam for three weeks in 2014: not one single coin in change, all paper down to the lowest denominations.
Coins from the 2003 series are sold above face value (but still for little to nothing) in tourist shops or on 'antique' markets.
The coins in 2003 serie are actually made of old Finnish 50 penniä and 5 markkaa coins as Vietnam ordered its coins from Mint of Finland. Order was over one billion coins total and it recuired ten boats to ship them to Vietnam.
Source (in Finnish only) says also that some other countries buried their pre euro coins into salt mines (!):