I've just come returned from a 2,700 kms ride & only rode in the rain for 20 kms.
Its early wet season & most of the rain seems to be coming from the west.
It's been raining at night, or early morning generally.
Towards the end of wet season the typhoons will probably start coming in from the east & the China Sea way so you'd expect Laos to be wetter then, being closer to the sea.
But you can pick when & where you ride a bit.
The GTR "Weather guide"
Southeast Asia Weather
For help deciding where the rain is coming from take a look at these satellite pictures.
1.
Thai Met Office.
2.
The Weather Channel.
3.
Foreca is an excellent phone app for the weather to try.
It only gets really bad when there's a tropical depression or typhoon that comes in causing it to rain for several days, otherwise as you know it's generally thunderstorms (but some are longer than others.)
If you are on the Xieng Khouang - Xam Neau plateau area then you are 1,100 metres + up & generally in the clouds at the height of the wet season.
I hope this is a help.