UK travellers can enter Vietnam for 15 days for business and tourism without a visa. Unfortunately, USA travellers cannot and will have to obtain a visa before arrival. To visit Vietnam for up to 30 days, it is possible to apply for an e-visa online. This costs £19/$25 for UK and USA travellers. The only problem is that this visa is restricted to certain entry and exit points, which must be selected at the time of applying.
For more flexibility, it is recommended to apply for your visa at the nearest Vietnamese Embassy. To visit Vietnam for longer than 30 days, a visa must be obtained from the nearest Vietnamese embassy before entering the country.