So basically we had heard lots of bad stories about crossing the northern crossings between the two countries, but since we were on a budget and didnt have all the time in the world we decided to go for it anyway.
The key part is off course the bordercrossing, and I must say it was much easier than expected. The only thing you need to do is get on the right bus. The bus leaves Phonsavan (get there by minibus/bus from Luang Prabang or Vientienne or whatever the day before) bus station at 6 in the morning and cost 136 000 Kip I think. We went to the bus station, and bought tickets there the morning we were going, but most locals just waved down the bus as it went back through the city and payed on the bus...
The bus leaves 4 days a week, find the days by google.
The reason why you want to take this bus is that it goes all the way to Vietnam (Vinh) 😊
You can make the trip with out taking this bus, but expect to pay more than the bus fare for just the moto down from the boarder to nearest town...
The boarder crossing was a bit confusing, but everything worked out well. We had to go the last leg cus some trucks blocked the road for a while, but still it wasnt any problems.
Find the lao boarder office, get your stamps in the passport, continue down the road across a bridge to the vietnam side and get the visa you allready got stamped and pay something silly (2000 VND or whatever leftover lao cash you have) for a health slip you will (probably?) never need.
You might find some customs that wants to check your bags, and then just get back on the bus.
Or wait for customs to go through all the junk other ppl got on the bus and see them fastening a live goat in a sack on the roof of the bus 😊
Hopes this helps someone, it really wasnt as scary as some ppl make it sound.
Feel free to comment if anything is unclear, or if you have some more info on this or other Lao-Vietnam boarders.
BTW: Did Bangkok to Hong Kong over land, and I would reccommend going overland if you got the time, you will see lots of places you would never see from a plane, and you will be much more flexible with dates and places.