From 1 July 2015 to 30 June 2016, all British passport holders travelling for tourism or business can enter Vietnam for up to a maximum of 15 days without a visa. The list of countries and regions that have visa exemption.
You must get a visa before you travel to Vietnam if you want to stay for longer than 15 days and/or if you wish to re-enter Vietnam within 30 days of your departure. If you plan to leave Vietnam and re-enter from another country make sure you get a multiple visit visa before travelling. Make sure you know before you travel the type of visa you need for the purpose and duration of your trip.

The most popular type of visa for Vietnam is tourist visa which is normally valid for 15-30 days. Tourist visas can be extended once you are arriving to Vietnam. Tourist visa is good for TOURISM purposes only.
Procedures:
In order to apply for a tourist visa online and pick up your visa at Vietnam airports, the following information is required:
* Your name on passport: it is your full name appears on your passport, name order is not important, so you can place either surname first or first name first.
* Your date of birth: in Vietnam, the date format will be DD/MM/YYYY
* Your Nationality or passport: the passport you are using to enter Vietnam (UK, US or Australian)
* Your passport number: as shown on your passport and should be valid for 6 months at least from the date of arrival.
* Date of arrival: it is not necessarily exact, you can fill in with proposed date of arrival, however, please be advised that you can just enter Vietnam later not earlier.

* Photos: 2 photos (4*6) are required at Vietnam airport only.

Related article:

Vietnam business visa

Vietnam work visa