Hi Jemima,
The Christmas departure is always popular - but then this is a popular tour throughout the year and we operate a number of dates that suit different people.
For the 26th November departure there are 17 travellers in total. 6 taking the main tour only and 11 of you staying on for the Cambodia add-on.
http://www.worldclimateguide.co.uk/climateguides/vietnam/ you can take a look at all the averages (temperature, rainfall, days of sunlight) for each of the cities you'll be visiting.
In terms of sun cream, insect repellent and kagool, I would say yes, yes and yes. You'll also want to make sure you pack a pair of shoes that are comfortable to walk in and a travel adaptor to make sure you can charge any batteries/camera at the end of each day.
Kindest regards,
Julia