We are in Vietnam 16 days from uk .. I'm taking it that we can't have the visa exemption and will need to apply for the visa ?
The day you arrive is counted as day one.
Doesn't matter if it's 11.00pm, or 1.00am, it's day one.
If the day you leave is day 16, then you will need to arrange a visa.
Ok , visa it is then , not able to change itinerary to make 15 days ...
Have a look at the E visa or VOA and see what suits you best.