i was in hanoi / sapa / ning binh / halong within 9 days, from 7 jan - 15 jan
weather was only cooling like genting during my stay there, there were days where the temperature suddenly just shoot up to 28C in the afternoon, i was in shorts n tshirt in hanoi most of my time, dun even need any jacket n or long pants long sleeve.
when i was in sapa for 3d2n, the3 days was very sunny, clear sky , no fog, the temperature there between day n night is very big, day time can be 15 - 16C , at night drop 5C . sudden sun set, 8pm onwards, the whole sapa become like freezer box.
in ninh binh, it has slight drizzling, temp was aroun 16C, day tour there following tour guide, so still ok,
halong bay was also the same, not cold nor hot, just nice, i was in a cruise for a night, nothing much to do except for some kayak, drinking night, and fishing.
overall, i would say, if wan top shop or food hunt, vietnam is not the place, thailand is better, if wan to see scenery , vietnam has much more to offer. i really like sapa due to the weather, scenery and less crowded n traffic. and people there are nicer, hanoi pple can be arrogant n rude.