Anetta raises a good point.
There is a conference centre in the USA called Bretton Woods and it was where, after World War 2 (German/Japanese) when the world was in chaos, nations of the world gathered and devised the Bretton Woods scheme of Monetary Management.
Boring? Sure, but it affects us all daily.
Basically, the US Dollar was a reference currency, most everything is quoted against it, Unfortunately, since the days of Bretton Woods things haven't gone so well and the bench mark of the Dollar has become somewhat changeable so there is movement both in country monetary values as well as the dollar.
Now there is the Euro, the Pound and, increasingly, the RMB/Yuan.
In order to get the best exchange rates banks have offices and even strings of banks, in the USA.
Someone in Europe wanting to transfer money to VietNam gets sucker punched twice. The Euro is exchanged for Dollars, the currency in which it is transferred internationally, then it is transmitted (as an international credit) to a bank in VietNam.
Then, usually, comes the second sucker punch. The bank happily exchanges Dollars into Dong, scoring another big hit.
The Dong is not internationally traded, they had planned to do this in 2009, but the Wall Street frauds screwed this up.
As long as the Dollar remains high against Gold, most everyone is satisfied.
But in recent years the Euro and now the Yen/RMB are frequently the medium of exchange - the Yen/RMB is so particularly in our part of the world.
The VN Dong can be exchanged in border areas of Cambodia, Laos and China but the rates are bad - especially from the women who wave money in your face at border crossings,
If you want to transfer sizeable amounts of money here, and you don't know any VN businessmen, you should use the SWIFT transfer system. Here sizeable means about USD$2,000 or more.
You may have noticed that the exchange rates on ATM withdrawals are pretty poor - for you. They are, on the other hand, great for the banks. Therefore, unsurprisingly, the banks choose the optimum time for them to 'exchange' your ATM withdrawals. Maximum screw rate.
With SWIFT things are a little different BUT it requires work. First, you have to inquire of your bank what procedures / forms / etc. they follow for transferring money by SWIFT.
Once you have instructed your bank that you will be issuing an order for them to send money to you, you then have to study exchange rates between the currency your money is held in and the US Dollar as well as the US Dollar/Dong rate.
You will also, preferably, have a USD$ account in their Correspondent Bank in VietNam. A Correspondent Bank has an agreement in place to look after the originating banks business. They can trust each other.
There are websites that will do this for your free.
At the optimum moment you trigger the SWIFT transfer. It has to follow a precise schedule - no HSBC fiddling the timing, it is fully automatic. It runs 24/7/365.
And within minutes.
The receiving bank will always say it takes a day or two. This because of time differences AND because, before they give you the money, they verify the transaction. A Correspondent Bank doesn't need to do this because of the inter-bank agreement.
Why is it handy to know VN businessmen?
If a VN businessman needs to buy something overseas (import into VN) he has to apply to the State Bank of VN to do so and obtain a currency purchase permit which will allow banks to sell him US Dollars.
However, where a businessman hereabouts has a pile of Dong and he needs to buy Dollars, he can arrange for you to transfer an amount of your Dollars (overseas) to whom he wishes to pay money and, when done, he will give you Dong together with a percentage added.
Bank >> bank internal transfers
Assume you have an account overseas with HSBC (now referred to as the drug dealers local bank in many newspapers) as well as an account here with HSBC, they will suggest they transfer the money for you, internally, i.e. NOT using SWIFT.
They make much more profit from exchange and you have less control. Even worse, they charge you way over SWIFT fees (around USD$15-17)
So use SWIFT!
Did you hear the HSBC was fined USD$1.9-billion for money laundering? Sounds like a lot? Not to the HSBC, it represents a couple of days operating profit!