Spent 2 days in Hanoi last fall.
Loved it, for all the reasons noted above. It was not what I was expecting, completely different from Hong Kong. And from most cities I've visited, for that matter.
Chatting with the university students around the lake was a great way to get an insight into what it's like to live in Vietnam.
I can't wait to go back, this time to both Hanoi and HCMC.
Flew Dragonair from Hong Kong.
Check Cathay Pacific--every week they post cheap flights to various cities. You have to sign up, and they send you an e-mail notification. Fares don't last long, but deals to be had.
I stayed at a small hotel near the Cathedral (Joseph's) and it was perfect.