I would recommend spending some time in Hoi An and less time in HCMC. Hoi An is a really pleasant place to spend a few days. I wasn't that keen on HCMC; Hanoi is a much more attractive city. The best thing I did in the HCMC area was a day trip to the Cu Chi tunnels which were fascinating.
I agree, you will have a better time in Hoi an for sure .
Thanks for the Hoi An recommendation - Thu, Ruckerthe, Bex76, Paulj... I still like to visit Hoi An but decided that I would do so in a second visit if that works out some day.
And yes, Anna and Thu, I plan to take a jacket with me for Hanoi.. though friends living there say that though usually by this time it gets cold there, this month it is still 22 C. Hoping it will continue so till I leave.