Take a bus from LP to Xam Neua - about 14-16 hours on a sealed road assuming there are no breakdowns or extensive road-work. Rented van will make it in a couple hours less (and wil cost about $300 - $150 to get there, and the same to get back (whether you're in the rental or not). The trip is gorgeous, and a stop at Phonsavan (just over m id-way) for a night or two to see the Jars is time well spent.
From Xam Neua there are morning buses to Vieng Xai - takes aout 45 minutes. One can stay in VX or take an afternoon bus or saungtheaw back to XN.
You'll have to ask your derriere about difficulty. It is sitting in a bus, which isn't too difficult...