Vietnam forum

Ive asked a similar question recently and no amount of reading or personal experience can really account for the weather during this time.
As far as ive been able to find out, Vietnam has three distinct areas and two major weather systems. One thing that seems to be a thread through everything is the temperature. In a word 'HOT!' very hot and very sticky due to the high humidity at this time of year. The rain in Ho Chi Mihn starts to creep in around June/July, this is a constant throughout the south, as you move up central coast this will relinquish especially as you hit the mountainous region as this shields a lot of the wet weather. Post July the North should clear up as the season shift switches although you are still likely to encounter rain.
On a positive note, you should find the rain comes in brief but heavy showers (this could also be a blessing as it clears the humid air) As with all rain wherever you are in the world the cloud comes first! so you should have plenty of time to find shelter, maybe take time for your main meal or a siesta, maybe do some 'housekeeping' catching up on-line with the folks etc. etc whilst it passes. Its also likely to be afternoon.
The seasons and the changes can be very varied by all accounts and no amount of studying rainfall or temp charts will prepare you until you hit the ground but read as much as you can before you go and prepare for all eventualities is my best advice.
At the end of the day, just remember it could be worse, it could be raining at home and you're not there! so enjoy...