Starting in 2018, using cryptocurrencies in Vietnam will be illegal

Well, this is why large denomination notes have been taken out of circulation...

There is a limited supply of U.S. dollars in a country like Vietnam, which naturally limits its use in illicit trade. Both paper money and cryptocurrencies have to be converted into "clean" money that can be deposited into banks to pay taxes and expenses. While paper money requires expensive and potentially dangerous laundering operations lest you end up with warehouses full of bills, you can hold crypocurrencies in a wallet for no cost until its needed to be laundered and exchanged for real income.

There's not really a sea of money changers here, in fact there's not many outside the backpacker/tourist area downtown. Also, the fact that the country has a few money changers is different that regular businesses accepting currency besides VND. Trying paying for your dinner with USD, I would imagine 80% would not accept it, and the 20% that do, would only do so if you paid at a rate that made it worth their while to go and change it themselves.

As an American living here, I'm all for it. The Vietnamese currency is far superior the American currency. All bills, no coins. Plastic bills that don't rip, and can get wet.