I think the best two words to describe the climate in Vietnam would be changeable and unpredictable.
The climate varies greatly from the south to the north with two distinct seasons;it's boiling hot and cold.
It's quite hand for us to forecast in the weather in Vietnam. Even in the middle of the summer you are still likely to get caught in the rain due to tropical storms.
At that time, there is torrential rain and the rainfall tends to be higher in the North than in the South.
So, when you first arrive in Vietnam, you should take it easy for the first couple of days to get acclimated.