Difficult to say as much depends on when the rainy season starts (normally in May). If the rains are early you could be in for mud in places but otherwise the conditions should be OK. Read up on some of the recent trip reports for Laos to get a better feel of the conditions of the roads in Laos.
We were there the last two weeks in May last year. Encountered heavy downpours in places but otherwise the riding was quite pleasant. Take some light raingear and just stop if it gets too bad!
Only travelled form Paxaan to Vientianne up to Luang Prabang and back with a side trip out to the Plain of jars. Not a lot of traffic except in Vientianne during "rush hour" late afternoon.
There is a fair chance of rain and over the last few days here, it has been hot and the rain heavy at times with pretty good lightning strikes. A good indicator of roads is the one I live on, it is not paved and a disaster at the moment. In general, this is the best time of year for mud hunting.
Drop me an email and I will try to help out where I can.