Nationals of the United Kingdom and a select number of European countries can enter Vietnam without a visa. This 15-day visa exemption is only valid for single-entry arrivals and is applicable to all border arrivals (regardless of whether you arrive by air or land). Your passport must be valid for at least 6 months following your date of arrival in Vietnam and have at least two blank pages.
For all other nationalities, multiple-entry visits, and anyone staying longer than 15 days, a visa is required. Your visa must be arranged in advance of travelling to Vietnam.
Citizens of 80 countries (including the United Kingdom, much of Europe, USA, Canada, New Zealand) can apply for an e-visa online. The cost is $25 per person and it is valid for 30 days. You can find more information and apply on the official visa website.