When I came on my first visit to Ha Noi 10 years ago it was late Jan - Apr, and terribly cold. For the last couple of winters it hasn't been bad. Today it got cold again, but in 6 weeks? Who knows?
Went to Vietnam during Chinese New Year last year...don't. It's the Tet holidays for them and everyone heads back to their hometown. The buses and trains are packed. Everything pretty much shuts down although some of the tours e.g. to Halong Bay are still operating.
Summer is HOT and HUMID - spring or fall might be more to your liking. Right now it is COLD, but that could change any day now. Spring and fall are definately my favorite seasons - except for the nice days in the winter and the summer, when it can be really pleasant as well.