Cambodia is slightly different to its enighbours, as US$ is seen as tourist currency.
In Vietnam only use Dong. It is actually a criminla offence for business to ask and accept foreign currency. You will often see prices shown in US$, but the only to use as "help" for tourists.
The only time you need US$ is if you have to pay for the Visa stamp as part of VOA. Otherwsie just bring GBP with you to exchange it into Dong. ATMs are also widely available
Dong is the official currency of Vietnam, and as such, that is what you pay for things with.
Bring your own currency and exchange when you are there.
You can also use ATMs which dispense dong.
A combination of the two is often recommended.
May do to have a read of this -
Also check the other 'Top Questions' to the right, at the top of the forum page.
Just bring your local currency (£/€/$) and exchange it to Vietnamese Dong (VND) as required on a daily basis.
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