Since last Sunday, the first north east monsoon has come to Hanoi & North of Vietnam. It brings cool dry weather to all of Northern area.

Weather Forecast of Hanoi (accuweather.com)

It will cold in the early morning and night time. Day time will be quite cool for everyone , especially with traveler from Western countries ( Who get used to live with the cold weather). Please notice if the winter come with the rain , it will be very cold . It is a unique cold feeling and it totally different with other winter weather. We highly recommend traveler to prepare , bring and wear warm clothing before and after arrived to Hanoi /North of Vietnam.

Northern Vietnam

Northern Vietnam has a cool to cold season (courtesy of the northeast monsoon from November to March) and a warm to hot wet season (from April to October). December and January in particular can get quite frosty in the far north of Vietnam -- pack accordingly. This time of the year can also see heavy mists that can run for days -- these can reduce visibility in places like Sapa and Ha Long Bay to frustratingly short distances. The wettest months of the year in Hanoi are July and August, the driest, December and January.

Ref: accuweather.com , travelfish.org