My husband and I have 9 days to travel around Vietnam in late December - it will be our first trip. Thinking of the following itinerary - should we skip Hue and fit in Sapa? Travel time to Halong Bay seems like a lot on a short trip but are we missing out by skipping it?

Day 1 - Hoi An (evening arrival)
Day 2 - Hoi An
Day 3 - Hoi An
Day 4 - morning departure to Hue (have read book 3rd class train on right side)
Day 5 - Hue
Day 6 - Hue to Hanoi
Day 7 - Hanoi
Day 8 - Hanoi
Day 9 - Haoni with evening flight home

Thoughts / recommendations welcome!

Well, I can say on the way to the north, you will go throught from Da Nang to Hue, I think it is okay if you stop for a city tour and stay for a night before the trip to Ha Noi, which is quite a long journey. Just check with a reliable company to arrange a private car and further service such accomomdation, tour guide.

Your above plan looks ok.
Between Hue and Sapa, which one appeals to you most? Cold in Sapa at that time.
I would prefer doing a private transfer from Danang (Hoian) to Hue.

Halong is a must, one of the most fascinating sites on Earth; So, yes, skip Hue and take a TWO-days/1 night on-board (magical !) cruise. Halong Bay is in fact 3 bays ; Lan Ha Bay near Cat Ba Island, Halong Bay north of LH itself, and Bai Tu Long Bay north of HB. Take a cruise on LAN HA BAY starting from Cat Ba Island (only 6-hours round-trip from Hanoi). DO NOT go to Halong Bay, very crowded and dirty, nor Bai Tu Long, which requires a 3-day cruise and do not take a one-day cruise, too short

For the Danang-Hanoi leg, you have the choice between flight or the direct NIGHT train (soft berths), which saves half-a-day

I agree with DinHo, get a private driver to take you between HoiAn and Hue and you can then visit the Marble Mountains and Hai Van along the way. Should cost around $50/60 and take 5/6 hours with stops. The train is nice but you clearly won’t be able to stop along the way.

I would forget about Halong Bay, it just takes too long in the context of the time you have. It is no longer "one of the most fascinating sites on Earth" as described in #4 above. Polluted, dirty and crowded. There are quiet spots, by frankly, you time would be better spent elsewhere.

Sapa is similarly crowded and, again, it takes along time to get there. There is more than enough to keep you busy in Hanoi. In fact, I would drop a day in Hoi An and add it to Hanoi.

If you wanted to add a destination and the train times worked for you, rather than fly form Danang to Hanoi, you could get an overnight train (an excellent way to experience Vietnam ) to Ninh Binh which is often described as Halong on land. ( I much preferred it to Halong) . Spend a day there and then continue on to Hanoi by train or car.
Some details and photos of our many times exploring Vietnam are in our blog @ https://accidentalnomads.com/category/vietnam/

Nothing on Halong but the other places inc. the train are.

I agree about Halong--- with such a short trip it is not worth it.

With a TWO-days cruise from Cat Ba Island, it is DEFINITELY worth ! Only 5-6 hours ROUND-TRIP Hanoi-Cat Ba including 2 hours RT Haiphong-Cat Ba-Haiphong on speedboat, with beautiful arrival in Cat Ba City. Not going to Halong when you are in the north of Vietnam, it's like go to Denver and forget the Grand Canyon !

MANY things to see and do in Tam Co. The boat ride takes only 2 hours, to be done in the morning before 11, when the groups arrive from Hanoi, or around 16, when they have left. Other things to do by bicycle (free at hotels) : Bich Dong Pagoda and Thai Vi Temple, both in Tam Coc ; Hang Mua Peak (superb panorama from the top - 3 km from TC) and the 2 temples of Hoa Lu (30 km round-trip bicycle or taxi. Stay at the famous hotel Chez Loan in Tam Coc, in the middle of the rices fields and limestone peaks (bicycles free and rental scooters 5$per day) ; she gives you a map of the area and explains where to go.

Tam Coc gets very busy with lots of domestic tourists, especially at weekends so I would head to one of the quieter docks to get a boat. We did a couple of different trips almond the various waterways - https://accidentalnomads.com/2014/12...d-kingfishers/
We were ending a hiking trip in Ninh Binh and so had a car and driver to get around. That said, much of our time there we used bicycles, a much nicer way to get around. It is not a huge place, but in order to get to the nicest areas a good guide would help a lot. We used Ethnic Travel. Easily arranged if you are coming from Hanoi and if you are arriving by train from say, Hue, I imagine you could arrange for a guid to meet you from the station. We don’t often use guides but this is one location where they can add value especially as you have limited time. We found ET to be very flexible but there are other operators too. Just stick to one of the reputable ones as there are lots of cowboys out there.

I think this itinerary is ok. Hue is a better choice since it fits this plan and Sapa is full of fog in late December.
It takes you at least 2 days in Halong Bay, so you may skip it for later. Add a point that, December is the winter in the North. You should travel to Sapa and Halong in April/ May next time.
Enjoy your trip in Vietnam

Thanks, all! Your responses are much appreciated.