Vietnam stretches over 1,800km up the eastern coast of the Indochina peninsula and has various climates at the same time in different parts of the country.
Winter season lasts from November to April in the North with temperatures averaging 16 degree centigrade. The rest of the year is considered summertime with heavy rainfalls and occasional storms.
The rainy season lasts from May to October in the south. However, it only rarely rains the whole day. It gets dry after rainfall as quickly as it comes. The dry season is generally from November to April.
The central parts of Vietnam are affected by the South West monsoon season coming from Laos during May.
June experiences a muggy heat and sometimes produces typhoons and tropical storms.
During November and December there is sometimes long lasting rains. Despite of this fact, the months of November and December are constantly considered the peak period for incoming tourists.
January and February are still occasionally cloudy and rainy.
The best time for a long Vietnam tour from North to South would be from January till April. For those who wish beach holidays can come to enjoy warm water and sunshine in the central coast during summer time, from May to October.

See also other FAQs

What is the best type of clothing we should bring?

Will it be hot and humid in most places?

How cold can it get in the winter?

How hot is that in the summer?

What is it this like to travel in monsoon season; will it cause trouble for us?