Houesai is on the banks of the Mekong, on the path between China and Luang Prabang, about 200 km from Luang Nom Tha, or 6 hours by bus.
My suggestion
If you get you visa for Laos here when you arrive from Thailand, you can pay the customs officers in Dollars or Kip. Be careful with the conversion because there is an astonishingly large difference between them.
Anonymous traveller
Seasoned Traveller
2
Houay Xay, a Laotian/Thai border town, is an almost obligatory stopover if you're heading to Thailand.
My suggestion
It's best to travel in groups so you can better negotiate the guesthouse and tuk-tuk prices!