Best way to get to Luang Prabang from Vientiane

The best way to get to Luang Prabang from Vientiane is to fly
There are 6 daily direct flights on this route, flying between 9am and 5pm
Flights take 40-50 minutes
The only two airlines on this route are Lao Airlines and Lao Skyway
Lao Skyway is usually the cheaper of the two, but their schedules are limited. Fares on Lao Skyway is round $50USD one way, and for Lao Airlines is $80-$90 one way

Alternative to flying:
There are lots of minivans and buses going between Luang Prabang and Vientiane
Travel times are 8-13 hours, with minivans usually being faster. Departures are all either before 9am to arrive in the late afternoon, or after 6pm to arrive in the morning the next day
All the overnight options are by coach buses rather than minivans. The overnight coach buses have lie-flat beds instead of seats (see pictures below), so they're actually quite comfortable for sleeping. However, these seats are usually designed for people 1.7m or under, so some people may find it very small
Fares range a lot from $22-$35 per person, with the overnight buses being usually pricier
This is what the inside of minivan looks like:
This is what the inside of day time coach bus looks like:
This is what the inside of overnight coach bus looks like:
In Laos, you can book your bus ticket through:
https://www.bookaway.com
https://www.baolau.com
https://12go.asia
Here's a list of all the major operators that I know of and the details about their Vientiane-Luang Prabang services:
PTS: coach bus with AC (the night option has beds instead of seats), 3.2/5 rating, two daily departures at 6:30am and 5:30pm, travel time is 11 hours for morning bus and 13 hours for the overnight bus, pickup service in Vientiane, arrival point is Southern Bus Terminal in Luang Prabang, no website, $30-$32 per person, book on https://12go.asia
Naluang: minivan with AC, three daily departures at 7am, 9am, 3.5/5 rating, and 7pm from Vientiane, travel time is 9 hours, boarding point is Northern Bus Terminal in Vientiane, arrival point is Southern Bus Terminal in Luang Prabang, website is http://naluangstation.com, $12-$17USD per person, book by messaging them through their website or on https://www.bookaway.com
Soutchai: coach bus with AC (the night option has beds instead of seats), 2.7/5 rating, two daily departures at 7am and 6pm from Vientiane, travel time is 13 hours, boarding point is Soutchai office in Vientiane but pickup service is also offered, arrival point is Northern Bus Terminal in Luang Prabang (across the street from the airport), website is http://soutchaitravel.com, $21-$25USD per person, book through their website or on https://www.bookaway.com
Naga Travel: coach bus with AC (the night option has beds instead of seats), 3.0/5 rating, two daily departures at 9am and 6pm from Vientiane, travel time is 12 hours, boarding point is Haysoke Guest House in Vientiane, arrival point is Southern Bus Terminal in Luang Prabang, website is https://nagatravel.com, $29-$32USD per person, book by messaging them through their website or on https://12go.asia
King of Bus: coach bus with AC (the night option has beds instead of seats), unknown rating, two daily departures at 8am or 8pm from Vientiane, travel time is 11 hours, boarding point is Phan Tha Vong Tours office (located here), arrival point is Southern Bus Terminal in Luang Prabang, no website, book on https://www.baolau.com, $22-$24USD per person
Thavisouk: minivan with AC, 9am departure from Vientiane, unknown rating, travel time is 8 hours, hotel pickup from Vientiane, arrival point is Southern Bus Terminal in Luang Prabang, no website, book on https://www.bookaway.com, $25USD per person