All good advice!
We did not love Hanoi after 3 days:
https://readyjetroam.com/2019/03/12/...ot-love-hanoi/
Check the weather before deciding on Ha Long bay...we were there in the foggy season.
We are currently living in Da Nang and it's a wonderful place to live but I wouldn't come here on vacation...seeing Hoi An and Hue is a better use of time.
Flights are cheap in vietnam:
https://readyjetroam.com/2019/03/14/...oi-to-da-nang/
As is lodging...a nice hotel in Hoi An will cost you no more than $35US/night.