Do UK citizens require a visa for Vietnam for tourist purposes?

Not if you are staying for less than 15 days and your travel takes place between 1 July 2015 and 30 June 2018 (and the scheme may be extended beyond this date, it has already been extended twice) as you can easily find out from both the UK Foreign Office Advice page or the website of the Vietnamese embassy.

If you are staying for longer than this (or trying to enter twice within 30 days), then there are two options for getting a Vietnamese visa:

1. You can apply to the embassy and either send off your passport, or send of an electronic form and take the passport to the embassy in person. This is the "safest" option and will get you in at any point of entry available to foreigners without additional steps. However, it can take some time and is often not the cheapest.
2. Alternatively, you can get a "pre-arranged visa on arrival". This involves paying for a company in Vietnam to get a letter indicating you have been granted approval to enter the country. On arrival you present this to a desk at the airport, and pay them a fee is US dollars per person (the fee changes more often than I want to change this answer). This option is only available when arriving in Vietnam at certain airports, and means after landing you have to go through and additional queueing process. Also, note that while many companies offering to acquire you the letter are legitimate, there are others who are not, and both the UK and Vietnamese governments refuse to identify which is which.

Do UK citizens require a Vietnam visa? Yes and No. :)

Yes when you stay longer 15 days; and No when you stay less than 15 days. The regulation just come into effect from this July. You can find more details about it here. There're 2 ways to get visa for British. You can pre-arrange a visa approval online through travel agent and use it to get actual visa later on arrival; or submit paper application in person/by post/by email at embassy and collect visa after 5 working days. I myself like the second option as it's more secure. This is official website of Vietnam Embassy in UK. Go in there to read more about procedures.

As of 1 July 2015, British passport holders travelling for tourism or business will be able to enter Vietnam for up to 15 days without a visa. If you’re travelling for business reasons you should contact the relevant Vietnam government department before travel to check whether any licenses are required to carry out your duties.