From what I have seen of Vietnam immigration, money talks and a few $$$ in the right pocket will soon solve any problems. If you are travelling with the visa approval letter, where the visa is then stuck into your passport on arrival, make sure you have a few passport sized photos or they will sting you as much as David Bailey would charge for taking them with their camera.
Make sure you get a receipt as well as there doesn't seem to be any check on visas in passports vs money received and the immigration officers are very reluctant to provide one unless specifically asked.
BTW I think Brits have visa free for Vietnam at the moment.
https://www.gov.uk/foreign-travel-ad...y-requirements