No visa needed for Vietnam and the last time we went to Cambodia it was visa on arrival for UK passports
British Nationals can enter Vietnam for up to 15 days for tourism, transit and business (but not paid or voluntary work) without needing to apply for a visa
For Cambodia tourist visas can be applied for online or obtained on arrival
Your statement has to be from the bank and stamped/verified. You get it at the same time as the letter. Immigration won't accepted a statement printed off your account