Hanoi and northern Vietnam have a distinct winter and summer season. A dryer winter lasts from November to April with average temperatures of 18 - 20ºC. Summer lasts from May to October and is hot and humid with temperatures around 30 - 35ºC. In areas like Sapa, prepare accordingly for the winter months as it has been known to snow on occasion. 

Hue and Danang, in the center of the country, have very hot, dry weather from February to August with temperatures reaching over 35ºC. The region can experience quite heavy rainfall between September and January.

Ho Chi Minh City and the south have a hot, dry season from December till April with average temperatures around 28ºC. The rainy season lasts from May through till November. It rarely rains for long periods, even in the rainy season, with most rain coming in short, heavy bursts.

You can see the current weather forecast here.