Indeed kepsimax! As much as I love Vietnam, and particularly its people, the widespread copycatting and fakery do make me despair sometimes.
I wouldn't say that the weather in Halong Bay is likely to be bad in February, just cold. Why don't you wait until you're in Hanoi before deciding? Halong Bay tours can easily be booked the day before.
Just out of interest, how many days have you allotted to Hanoi itself? Three are the minimum required to get a feel of the various faces of the city and see the most popular attractions.
Sorry, I've never stayed in Danang so can't recommend for or against. Hoi An is far from being my favourite town in Vietnam, but most people seem to love it so you might well enjoy having an extra day there. It probably won't be warm enough to swim or laze on the beach, but there are plenty of other things to do in the area.