VISA Application

We fly from Phu Quoc to Ho Chi Minh City on the 15th of January. We leave the next day (16th of Hanuary, staying overnight in Ho Chi Minh) to return to Scotland. We have already been in Vietnam for a week before flying to Phuc Quoc from Cambodia (stayed in Vietnam from 28th Decmeber to 2nd of January) do we need to apply for a visa, if so can this be done online? Please can you advise.
We are British citizens. We didn't realise that the 15 day exemption was only for 1 visit, we were under the impression that it was 15 days in total regardless of the number of times you left the country.

Thanks