There is a report on this area made by David/Mai in January 2008. Here follows the part which may be of interest to you:
Rt10 and heading South East – Once passed Mouang Khoune we continued South East towards Phouviang. This is not to be confused with the route heading towards Xaysomboune or Pakxan.
Phouviang is actually a very pretty valley setting at the confluence of multiple valleys all coming together. The road was a nice hard dirt with some rock. There were also multiple nice dirt roads and tracks we could see that radiated out in all directions as we went South East. We were told by the locals that from this valley, you can continue Southeast towards Lak Xao in Southern Laos, but that the road was not very good and takes two days. Also there are many places to get lost and off the main road/track and various unmarked forks angle of at odd times.
The road to Phouviang is actually quite good and doable in a small truck. My Gecko map for Laos shows a village called Mok at the end of a trail going south-east from Phouviang but I don't know if this Mok is the same as your Mok Mai and also don't know if this is manageable in a truck.
A Google search shows Muang Mok Mai located at Latitude (DMS): 19° 15' 0 N and Longitude (DMS): 103° 45' 0 E which would be about the same location as Phouviang while the Gecko map puts Mok at 18 55 N and 104 0 E.