In Hanoi & the north, May to October is hot and humid with high rainfall while November to April is cooler and dry. In the Far North ( the mountainous area such as Sapa, Ha giang, Mai chau… ) December & January can be particularly cold with the temperature drop below 10 degees. It can be said from November to April is the best time to visit the North Vietnam. Central Vietnam ( Hue, Danang, Hoian ) experiences hot, dry weather between January & August when temperatures can hit the mid-30°C’s, especially high in May, June, July and August, can reach 38 degrees while the high levels of rainfall can occur in September, October, November and December. In Ho chi minh & the Southern Vietnam the dry season lasts from November to late April or May, and the rains from May through to October. Since most rain falls in afternoon downpours, this is not a big problem, though it may cause flooding in some areas in the city, make transport more complicated. The hottest months in Ho chi minh are April, May , June, July and August. The day time temperature can be around 37 degree but it is cooler at night at around 28- 30 degrees.
In summary, it is hard to tell you exactly when is the best time to visit Vietnam. But for the Euro market, we see almost people come to Vietnam from November to April. The low season may last from May to September. But it depends on where you are from. For example, the people from Belgium and Holland intend to travel to Vietnam in June, July and August- the hottest months.