The distance of the trip is 688 kilometres by road and can take between 12-15 hours by bus or around 4 hours by plane (via Udon Thani).
How To Get From Chiang Mai To Vientiane, Laos, VIP Bus or Plane Thailand
There are two ticket types which can be purchased, standard tickets and VIP tickets. VIP tickets need to be booked in advance as they do sell out, and general tickets should also be booked in advance during peak tourist season. The VIP ticket costs around 600THB per person but if you go through a booking agent, it may cost a little more.
If using a scooter to get to the bus station, there is overnight parking available for a small fee, or a Red Truck (Songthaew) will cost around 40THB per person or 100THB for a tuk tuk (per ride).
The buses leave at around 6pm each evening and are in the form of mini-buses which come with air-conditioning and recliner seats. VIP tickets will also receive bottled water and a snack for the trip. Several stops will be made along the trip for toilet breaks, but note that if making the overnight trip, there may not be many options to purchase food on the journey, so make sure to bring your own.
You will arrive in Nong Khai which is right near the Thai/Laos border and will be transferred to another bus to make the trip over the border. You will need to be processed at the border for leaving Thailand (make sure your departure card is filled out) and will then cross over to Huay Xai in Laos where you will be processed again by Laos Immigration. The Laos Visa prices is $35 USD, so you will get the best rate if you pay in USD. Thai Baht or Euros.
In addition to the bus, you could also take a flight from Chiang Mai to Vientiane. The ideal route is a flight from Chiang Mai International Airport to Udon Tani Airport. Shuttle services available at Udon Thani will then take you to Vientiane, a short trip that takes a couple of hours (including visa procedures).