Road transport
Laos is a country where much remains to be done in terms of transport management. Lacking budget, the country is also largely mountainous, so the roads are winding or, failing that, longer and more expensive to build. There are therefore many unpaved roads as soon as you leave the main roads and roads undergoing endless expansion and therefore dusty.
As a result, on average, it is necessary to count 5 hours of road for a journey of 150 km, a journey which, in Thailand for example, with better infrastructure, would typically take 2 or even 3 hours Max.
But there too, Laos is trying to improve its network, with the help of neighboring countries (China in the lead) in order to improve trade.
Thus, no less than 6 highways are currently planned, most of them starting from one border to join another on the other side of the country.
An example being the one, partially in progress. A highway is under construction between Vientiane and Catch Vieng. The latter, 30% complete, should open by 2021. This will reduce the journey time to 1h30 from at least 5h at present.
Eventually, just like the train line, this road should reach the border with China at Boats.
The buses
There are no specific companies I can recommend, as I just took the bus every time, going between this city and that.
The choice being mainly on the schedule, we still distinguish two types of transport: local buses and so-called “VIP” buses. These are often buyouts of Chinese or Korean buses, so old models, but not rotten either and therefore more comfortable than local buses.
If you do any long journeys, there are bus berths. An example, the journey luang Prabang - wow Xai, which I took when returning from the cruise (see below). It is necessary to count 12 hours to cover the 475 km, and this can be done at night.

Night bus between Luang Prabang and Huay Xai.
The shortest journeys can be made with mini vans. (this was the case between Vientiane and Catch Vieng withaller).
It is possible to book trips, either via 12go which I offer below (and earns me a welcome commission), or via this site, which I have not personally tested but seems reliable for popular destinations: https://www.laosbuses.com

Bus terminal in Thakhek.
Tuk-tuk or samlo
In the local version, we find our friends on 3 wheels or other homemade “sidecars”. And as all over is hard to know the exact rates, on the other hand it can be easy and economical to share your journey with other people to reduce the cost. A common case is to reach the city from the bus terminal, often a few kilometers outside the city, as if on purpose...
A concrete example, between the bus terminal just after the border at Huay Xai and the center, 7 km away, you are asked for a minimum of 80 baht per person.
Another example being the tuk tuk after the border between Mukdahan and Savannakhet. Typically they will wait for you just after the border crossing; whereas the Mukdahan – Savannakhet bus, which normally goes to the terminal which is well into town, often (intentionally) omits to wait.
And there, they don't hesitate to charge you up to 200 Bahts to go to the Thai consulate (which many expats like me do), to go 4 km. Speaking Thai (very close to Lao), it is generally possible to ask for 100 Bahts. A more realistic rate would be 60 Bahts.
Motorcycle rental
As everywhere in Asia, Motorbike rental is a convenient way to get around independently and popular since it is easy to get a two-wheeler and for next to nothing (generally around 70 Kips, or €000).
However, if you will rarely be asked (not to say never) a license to rent, it's not something I particularly recommend. Get an international license even if the latter is not sufficient since to drive in full compliance, you must have a valid A license in good and due form.

My little petrol boat wandering towards the Thakhek caves.
The risk is not necessarily the fine itself, but rather the fact of not being covered in the event of an accident, regardless of your insurance (who will cover you if you drive illegally??).
If you are a biker and are looking for good, well-maintained motorcycles for a trip to Laos, a good address:
http://www.rideinlaos.com/fr