I don't know if it is offered, I personally would just take the bus. That road is fairly windy in places and the buses and other vehicles fly through there so you would have to be careful if you did it.
I would just wait and rent a motorbike in Luang Prabang, it is quite a nice ride to go down to Kuang Si Falls and there are other places to go check out as well. In Vang Vieng there is also an expat who offers dirt bike riding, I was going to go try it out but I ended up skipping Vang Vieng due to Dengue Fever... You can find his info on tripadvisor if interested.
or read through this thread (note some of it is outdated)
It will probably cost about $35/day for a decent 200cc+ bike and another $50 for one-way drop-off. I've done the ride a few times and it's just great having your own wheels, and most of the way traffic is light. Get out of Vientiane and it's chill time.