There are a number of outlets in the U.K. where one can purchase Dong. A good comparison website can be found here at CompareCurrency website. Currently the best rate is to be had from MoneyCorp whose website can be found here.
Yesterday at Marks & Spencer (in St Albans, Hertfordshire) you could purchase Dong for cash ‘over-the-counter’ for quite a good rate – around UK Pounds 314 was buying around 9,550,000 Dong – rather better than orders from Treasury Vault in the USA. and, as Steve says, no additional charges.
I cannot offer any advice for European readers outside the U.K. I have looked on the Continental sites for Travelex (France, Netherlands, Germany and Switzerland) and the Vietnamese Dong is not listed on these sites, unlike the U.K . Travelex site. I suggest that European readers do their own Google search for Dong outlets in their own country, but major airports currency exchange facilities might be a source.