1 September 2017 - Since 2 years there is a persistence story that comes back in Thai and Lao news .... a straight bus from Nan to Luang Prabang (Laos). Via Huai Kon International Border. Until now this has never materialised, but the latest news is that it is now really happening soon. It would be great thing for Nan as it would be Nan more attractive as a holiday destination.
The distance between Nan and Luang Prabang is 367km thru the mountains. And it is said to take 6 hours, what I doubt. I think 8 hours is more realistic. And although you could get a visum for Laos at the border it would take about 30min. So I would think the bus company will require that you have already a visum for Laos. Thai people do not need a visum so that is easy. For returning back into Thailand I think you can not get a visum at this border, but maybe they will change that. So you would get a Visum Free Entry Stamp, 15 days or 1 month depending on your nationality.
As this route goes thru the steep mountains of Thailand and Laos there will be used a small bus with only 17 seats. Target price for the trip will be between 500 and 700 Thai baht. There is not yet a date set for the starting of the schedule.