Phoodoo (Pudu) is a remote post with very little activity; I passed through many times with my car. Latter can slow down the process considerably as a temporary Thai export permit, a DG53 (same on the Lao side for the import) plus the passenger(s) arrival/departure cards have to be processed.
If you are very organized (I have all those papers in the computer to be printed out as a complete set; add date and signature and Bob's my uncle), you are through in 20 - 30 minutes. The road from i.e. Uttaradit to Phoodoo is rather remote, so is the road from the Lao border via Kengtao upto Paklay, where you cross the bridge and carry on to Xanakham and Vientiane, a partly bumpy road on lone road grid.
Bottomline is, do the entire trip CNX-UTH (Udon) on Thai territory by bus, get a bus from UTH straight into Vientiane (if you have a Lao visa) or a bus to Nong Khai Friendship Bridge, do the Lao visa and carry on into Vientiane proper by local bus. One day per leg is gone for sure. Doing the trip via Phoodoo by public transport is a "mission impossible" but nothing became possible if not someone tried the impossible ???? As long as the journey is the destination - that is!
Good luck!