The journey from Vang Vieng to Luang Prabang took around 6 hours although the agencies estimate 5 hours.
I just went into any travel agency to book a bus ticket. The prices are really similar so there is no need to go in search for the best price.
The views from Vang Vieng to Luang Prabang is amazing. Tons and tons of lush green forests spread out in front of you.
The bus that I got on made one stop half way for bathroom trips. You have to pay for using the bathrooms though. Its great to just go out and stretch your legs and body from the bumpy ride. The bus drivers are so skilled. They don’t slow down at all swerving sharply on the mountainous roads. I felt like my life was on the line some of the times because the roads were very narrow and all you can see is a cliff right next to you. But the driver never slows down! Guess that’s why we arrive on time at Luang Prabang haha. Phew!