Yes. You should comeback Noi Bai airport and then fly to Danang city, and take a taxi to Hoian.
I think you should spend 1,2 nights in Danang city.
You can travel by mini-van or private car from Hanoi to Halong Bay on the morning of your overnight cruise. Normally the boats depart at 12 noon and return to port at about 12 noon next day. I think a one night cruise is enough. You would then have time to transfer from Halong Bay to the airport for your flight to DaNang. The people you book your overnight cruise with can easily organise your transfers from Hanoi and return to airport.
Thanks you so much for your help. My only concern was our luggage as we were told we just go out to the boat in small tenders.
Thankyou for your help.
Overnight in Halong is an excellent choice for you. You should book a private car for whole tour or at least transfer in and out of each region. It makes you not be worried and not expensive. You can enjoy one night in Halong Bay then transfer directly to Noibai airport and flight to Danang. The car in Danang will take you to Hoian. Or you can transfer by shuttle bus of the Cruise then take a taxi to the airport.
Let enjoy your wonderful trip in Vietnam!