From Hanoi to Ho Chi Minh is some 1600km which gives some indication of the size of the country. As a result the weather in Vietnam varies according to distinct climate zones. Due to regional climate differences there are no good or bad seasons to visit Vietnam. When one climate zone is wet and cold another is dry and warm.
For the purpose of this summary of Vietnam weather we'll consider the country as three separate areas:
Northern Vietnam (from Hue up North) has four seasons with a cold winter and hot summer. The temperature ranges from 5-6C in winter to up to 35C in summer. The further North it is, the colder it gets. In mountainous regions in the far North (such as Sapa) the temperature sometimes drops below zero. Rainy months are August, October, November. February and March have persistent, light, drizzling rain.
The best time to visit the north of Vietnam is from September to December when it isn't so humid. The weather turns quite cold and wet in January and this continues until March. The humidity can prove oppressive from May to the start of September and there's a danger of flooding.
Temperatures are high all year round. The temperature tends to be warmer and more humid than the South in the middle of the year and cooler during the dry season from November to April. Flooding may occur from October to December.
December to April is the best time to visit the south of Vietnam. There are short downpours during the wet season from May to November but they tend to only last a short time so you can still enjoy a visit during these months (just go for lunch when you see the black clouds developing). The Mekong Delta can experience severe flooding during this period making travel very difficult.
Has a distinct dry season (from November to April) and wet season (from May to October). Average temperature ranges from 25-30C.During wet months, heavy and short downpours may occur in mid-afternoons.
December to March is a good time to visit the Central Highlands when conditions are usually dry and cool. We visited the region in late November when the weather was beautiful with fine, warm days and pleasant evenings. We had a few light showers as we headed north towards Hoi An. Along the central coast there is a rainy season from December to February. June to October tends to be very dry and hot.