Vietnam visa for UK citizen

For the first leg of your journey, you're good to go because for visits of 15 days or less, the 'visitor' rule is engaged...

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.

For your return journey to Vietnam, a different rule is engaged...

You’ll still need a visa to enter Vietnam for periods of 15 days and longer and if you wish to re-enter Vietnam within 30 days of your departure.

Because you are returning to Vietnam after two weeks, you will not have cleared the 30 day cooling off period. You will have to get a visa or stay in Cambodia for at least 30 days.

The standard advice for your predicament is...

The safest option is to get a visa from the Vietnamese Embassy before you travel. If you plan to leave Vietnam and re-enter from another country make sure you get a multiple visit visa.

All citations in this answer come from Vietnam: Entry Requirements (The Foreign Office)

Updates on June 18th, 2015: Nationals of certain countries may visit Vietnam without visa for a period of time. Starting from July 1st, 2015 to June 30, 2016 visitors from the following countries may not need a visa to Vietnam for less than 15 days: Germany, France, United Kingdom, Italy, and Spain. First time you don't need a visa. Second time if you want to come back Vietnam which don't need visa you have to wait after 2 weeks.