There are several ways to get from Bangkok to the Lao capital. I found the overnight sleeper train from Bangkok to Laos the most reasonable option.
Why?
The Night train from Bangkok to Laos is cheaper than flying! A lot cheaper, actually. The cheap airlines like Air Asia haven’t decided yet to fly into Laos so you pay at least 230$ one-way with Lao Airlines. The others are even more expensive. For the night train you pay around 700 Baht (~23 US$).
It’s more convenient than a crowded bus.
Bus will be a little cheaper but it’s a lot more exhausting. In a train you can walk up and down, the bathrooms are so much cleaner and actually pretty nice. And I personally find going by train smoother than by bus.
You save the money for a hotel. As the train leaves at night and you get to sleep in a real bed and not just on your seat you get a pretty good night sleep and save one more night in any hotel in Bangkok.
Last but not least. It’s a nice way to meet other travels heading to Laos in the same train!
Also check out this post to get more information about how to actually do it.