We have settled on taking some cash VND and USD. The VND we have obtained from M&S are new unfolded notes.The Cambodian Riel is not obtainable as already pointed out on this thread.
We have also obtained a ‘Revolut’ plastic card as recommenced by some friends who have travelled to Vietnam and Cambodia recently. The Revolut card is one of those put on UK pounds via a linked UK bank account via an app and then use it locally at retailers or atm machines to get local cash. The card can be frozen on or off via the Revolut app (nice security feature we thought) and can be set up so that transactions only go through if the associated mobile phone is in the near vicinity of the credit card - we thought that was a useful additional security feature too.