Dear Ben Cooke,
Thanks for your questions
If you are coming with UK passport holder, you no need visa for staying 15 days. However, the previous visit (by visa exemption) must be far at least 30 days the next visit and your passport valid at least 6 months.
In case you don't have enough condition above, we can help you obtain visa https://www.vietnam-evisa.org/apply-visa.html