One day more in Vietnam North or South?

When are you going? Temperature/weather would probably be my highest consideration for that choice, but since you have more time in Hanoi I'd probably give the extra day to HCM. Depending on what your 1.5 day tour covers, you could do some other kind of organized thing (scooter tour, day trip to Củ Chi tunnels) or just chill. I found the HCM traffic very overwhelming though.

But if you could swing it instead, I'd apply your extra day to do an overnight trip to Ha Long Bay - it's a real long drive from Hanoi out there and having the overnight makes it less of a slog.

I wasn't enamored with HCM city, to be honest, so my vote would be Hanoi, but I do agree that if you could swing a night or two in Halong Bay it could be lovely.....though I think you are adding this to a Mekong cruise?
If that doesn't appeal, maybe a day trip to Ninh Binh. The karst landscape is lovely. I also liked the small boat ride through the rice fields and caves. However, look for rowboats with some padding. The trip can be hard on your back. Alternatively, forego the boat, stay in the village and enjoy the waterside walks and restaurants in beautiful scenery.
If there was a way to do HoiAn as mid way between the two cities, that could be a nice change to the big cities. You fly into De Nang. flights are pretty fast and inexpensive.

Thanks CL and Marie. I was thinking maybe an overnight on Ha Long Bay instead of a day trip. Maybe we could do that for our last night and have our transfer straight to the airport since the Paris flight doesn't leave until 11:30 pm.

I spent five weeks in Vietnam a couple years ago. I liked Hanoi better than HCM City, although there’s plenty to see and do in both. Depending on the time of year, Hanoi’s weather might be a bit more pleasant. HCM is pretty hot and humid year-round.

It’s a really long drive to Halong Bay. If you do it as a day trip, you will have only a short time actually cruising the bay. This means you won’t see some of the nicer and less crowded parts of the bay, and you’ll spend a lot of time just driving. I would highly recommend you spend a night out on the bay on one of the many ships that offer this. Your idea of doing this on your last night and then heading straight to Hanoi’s airport the next day sounds like a possibly good plan. You could have a leisurely day before your late night flight. I think it takes about four hours to drive to the airport from Halong Bay.

I like the idea of making your way to the airport from Ha Long Bay, a nice relaxing last stop before a long travel day. Even though your flight is super late give yourself a generous cushion to get to the airport, in my memory the roads were super terrible, under construction too, and it took twice as long as you thought it should to get there. Maybe things are better now, you might check Trip Advisor or Reddit to get current status.

I agree with MariaF - if you could swing it, Hoi An between the two larger cities would be a really nice change of scenery. You'd have to fly into DaNang and hire a driver to take you to Hoi An.

I vote for a night on your Ha Long Bay cruise (I spent 2 nights and really enjoyed it, even though some don’t). I really wouldn’t do it as a day trip.

But I also loved Hue (fly to DaNang?) - even more than Hoi An. Might not be as practical, though.

Edit: My cruise would drop me back in Hanoi, but not the airport. It would have been more convenient for me also! So I took a taxi from the hotel I stayed at before the cruise, since they returned me there.

“ I was thinking maybe an overnight on Ha Long Bay instead of a day trip.”
Go for it.