Not off road but a good half day loop ride out of Vientiane with mix of bitumen, fast dirt and washed out forest roads.
VTE - Samphanna - Turn right to Phialat - to Souanmon - turn right to KM52 and Rte 13 North and right turn back to VTE. (names as spelt on David's map)
Head out of VTE on Luang Prabang road and go straight at the fork - turning right goes to the north on Rte 13 (the Nissan showroom). This road turns to dirt after a few kilometres and follows the Mekong. At a Y intersection (a service station is on the right) take the right fork (signposted 17km to Phialat - it is 21KM to Phialat the other way but there must be another turn somewhere because I have been down the left fork and not ended up in Phialat). Anyhow the signs are all in Lao script so where they say they go is probably not relevant to most. This is a good fast dirt road, only needing to slow down for villages and cattle. I chose 80km/h as it was fast enough to enjoy and also slow enough to slow down quickly if I had to. I'm sure you could go faster if you wanted to.
You reach Phialat exactly 17km after turning (you know you're in Phialat because there is a sign on an Army camp in English).
Keep going another 18KM (signposted in Lao) to Souangmong (Souanmon on David's map but the Lao spelling has the ng letter i think).
There is a T intersection in Souanmon (a concrete 2 story house is on the corner) take the right turn or go straight ahead. Right turn goes through the NBCA over to RT13 - going straight will end up at RTE13 at Houay Ileuth a bit further north. I turned right and the road quickly deteriorated from fast 80km dirt to slow single track. The most challenging part of this road are the bridges made of logs running parallel to the road which means riding across the top of a log and not slipping off into the wider than wheel sized gaps on ether side. There are 4 or 5 of these - i lost count and didn't bring my camera to take any photos. If anyone with a R1xGS or KTM9x0 plans to try this road I will certainly take my camera to capture the moment!
I was relieved to see Rte 13, hitting it at KM55 just south of the toll booth. Turn right into KM52 village which is about the only place to stop and get a cold drink before the crappy death cheating ride back into Vientiane. I was only forced of the road twice this time by ignorant pick-up and landcruiser drivers driving on the wrong side of the road.
Total distance is about 160km and it took me around 4 hours. While I travelled today (July wet season) I think the road would be impassable if it was raining or had recently rained.