We had an amazing holiday there about 4 years ago, kids were 12,10 and 7. We had 3 days in Hanoi (you get there late in the day so need the extra one). Hanoi's roads are mad, you want to stay somewhere near Hoam Kiem lake near the weekend as they shut the road and it's not quite so manic.
The water theatre is great - needs pre-booking and also the museum of Ethanology where you get a glimpse of the different types of dwellings that the many different ingenenous groups inhabited over the last couple of centuries. We enjoyed the military museum as well.
We then travelled to Halong Bay area for a cruise - we were actually more off the beaten path, but nearby.
Then to Hoi An - we stayed in the Angsana hotel which wasn't in the centre, but on a lovely beach. We did a group cookery course and market tour in Hoi An which the kids still talk about..
Flights are easy, but I would book yourself, rather than through an agent as we got bumped, which wouldn't have happened if I'd booked them myself.
What I would say is though, that I wouldn't do it at the moment. Vietnam is a communist country and things change quickly and arbitrarily. Our old au pair's brother contracted COVID there last year and was forcibly quarantined in an old mental asylum for 3 weeks.