You should apply for a visa if intending to stay for more than 15 days OR not meet the requirement of “a gap of at least 30 days between two visa-free visits is required”.
Under a government directive released in January 2017, citizens from 40 countries will be eligible to apply for the e-visa (30 days, single entry), including citizens of the United Kingdom, therefore, UK passport holders can apply for an e-visa if you want to stay in Vietnam for around 30 days.
In case you want stay more than 30 days or want multiple entry visa for Vietnam, you are advised to choose to apply for visa on arrival. There is a visa issuing office located at the international airport where you can show the visa approval letter (pre-arrangement) to pick up your visa there.
Please note that unlike visa on arrival of some countries in the world, in order to get visa on arrival of Vietnam, you must have a visa approval letter from the Immigration Department of Vietnam which allows to pick up your visa on arrival at the international airport of Vietnam.
If you want to pick up your visa on arrival at international airports of Vietnam, you should apply online through a website for Vietnam visa or contact your travel agent in Vietnam to ask for a visa approval letter from Vietnam Immigration Department.
Then upon arrival at the international airports of Vietnam, you are required to show the visa approval letter to get visa stamped on your passport.