"How can the drops of water know themselves to be a river?" - Zen saying
Unlike much of the rest of southeast Asia, the weather in Vietnam can vary depending on which part of the country you are in. In general, it is hot and humid but the temperatures and the rainy seasons will differ.
In the north, the winter season is from November - April with temperatures averaging 20°C (68 F). January through March tend to be the coolest months. May through October is summer when temperatures and humidity are highest. July through September sees the most rain.
Central Vietnam ( Hoi An, Hue & Danang ) experiences hot and dry weather from mid-January to late August. Temperatures can often be in the mid 30sC (low to mids 90sF). During the winter months the rainfall is highest with October and November being the rainiest months.
In the southern central region, NhaTrang has a slightly longer dry season (January to September), with high temperatures and little rain . The rainy season runs from October to early December.
The weather in South Vietnam is most similar to that of Cambodia and Laos. The dry season runs from November through April/May with the highest temperatures and humidity occurring between March and May. The rains begin in mid to late May and continue through November with June through August receiving the heaviest rainfall. It rarely rains all day and it's most common to experience a mid to late afternoon rainfall of an hour or two.
Temperatures in the south are consistent year round and fluctuate between 25°C - 35°C (77F - 95F).