I agree with the above posts.. Get a guide and may be do a bit of a ride out and around Luang Prabang or another locations.
After 5 days to a week if you feel comfortable then take off on your own down to Phonsavan and Vientiane etc.. where ever you have planned.
The safety training can't hurt.. But many of the things they will teach you not to do is what the Laos will be doing..
A lot of the roads don't have crash barriers, potholed, gravel etc.. and you may come around the bend and find a truck in the middle of the road coming towards you. he will not move.
Once I was coming around a bend, a truck was in the middle of the road and a bunch of goats on my right about to cross.. Luckily they moved back with the noise.. Lao drivers will over take in your lane and push you to the right side of the road.. another time we had a cow jump out of some bushes and run across in front of us.. 5 metres closer and it would have been one of us.
Make sure you wear good safetly gear too as you may well come off if you are not experienced. (even if you are experienced

Boots, body Armor, Gloves etc etc.
Not to put you off.. but recommend a guide first to get some experience with the conditions etc then take off on your own.
You will love it. Have a great trip.
Cheers
Brian