On holiday in Vietnam and going to Cambodia on Sat. Can we use Vietnamese currency in Cambodia
Why would you expect that?
NO. Not directly.
You will need to use US$ and your small change will be in Cambodian Riel - KHR.
VN Dong is internationally convertible.
Cambodian KHR is not a convertible currency and totally worthless outside of Cambodia so please spend it as you accumulate it at the constant "street" X-Rate of KHR 4,000 = USD$1.
Best regards. Ian/Angkorlad