Hi Steve,
I did this tour in Feb 2013 and had a brilliant time! It will be cooler in Hanoi and Halong Bay than elsewhere so bring a fleece/rainjacket just in case. It was misty when we were in Halong but we still had a great time cruising between the islands. It was hot from Hue southwards and pretty humid in Ho Chi Minh City. The food was excellent, a lot of spring rolls and fish and very tasty, not too spicy. The hotels were very good throughout and the tour was excellent from start to finish.
Regarding the flight, hopefully you will get at least a few hours sleep in between the meals. Watching the tv may help to send you asleep but for some people the bright screen keeps them awake! Perhaps you could bring a book as well. Some people prefer an aisle seat so they can get up easily to stretch their legs/use the toilet whilst it can be nice to have a window seat so you can snooze hopefully uninterrupted - you could also have some nice views, though this isn't guaranteed! If you have a preference, ask for it when you check in - no one likes the middle seat! I've been on a couple of hundred flights by this stage and I still get a bit excited/nervous beforehand - but at the end of the day, it's just a means to an end: a great holiday!
Hope this helps and if you have any other questions, just ask. I'm sure you will have a fab time. And enjoy Christmas in the meantime!
All the best,
Bob