When you're in Hanoi, definitely arrange a tour with HanoiKids - one of the highlights of my trip. It's a free tour of the city with a university student who wants to practise their english. Well worth it and you end up having the best conversations and seeing the city with someone who knows it inside out.
Also set aside a few days for Hoi An as it is absolutely incredible. So chilled and relaxed in comparison to some of the other places you're heading. I could have stayed there forever. If you like a drink, try the mojito place near the river which was exquisite.
Vietnam's an amazing place - you'll love it!