All the countries of Southeast Asia are subject to monsoon climate in which Laos and Cambodia have similar climate to the South of Vietnam, where it is warm and a little bit hot all year around. But the North of Vietnam experience a winter when it can be dropped up to -2 degree. In general, South, Central and North of Vietnam have different climate conditions.
Vietnam is known as 3/4 of water and a long coastline of 3000 km so its climate is varied significantly from North to South in which Northern’s temperature is around 10 degree when winter comes. But the Southern and Central is hot almost all the year.
In the North of Vietnam, the rainy season is about 5 months from Mid May to September. In other two regions, it ends in November, but the central will be affected most by the rainfall and rains may last for days while it is only short downpours in the South.
In general, there is no bad season in Vietnam thanks to its diverse, Everything ultimately depends on your interest to travel with cooler or warmer or will experience both.