It is easiest but most expensive way to come.
1) You come from Suvarnabhum airport in Bangkok to Vientiane Laos. You can take Thai or Lao air.
There are flights at 19:00 and 20:00. It only takes one hour to come to Vientiane. The plane schedule changes frequently so you need to check the flight schedule before your reserve.
2) You can come from Donmuang Bangkok to Udonthani.
There are Nok air and Thai air at the Donmuang. Check bulletin board number 3 for the flight schedule. You can check specific time for the flight schedule.
3) Is there flight from Suvanrnabhum Bangkok to Udonthani?
Yes, there is. You can take an Air Asia. This flight goes straight from Suvanrnabhum to Udonthani. The arrival time at Udonthani is 17:50. You need to check the home page of Air Asia because the flight schedule is very fluid.
You can take a train at Hualamphong to come to Nongkai. I recommend you to reserve for the lower seat which it has an air conditioner or fan. It will be cold in a room with air conditioner if the weather is not that hot like in dry season. I recommend for a lower seat because you might get carsickness if you take an upper seat. Please refer to number 8 of the bulletin board. There is price and train schedule. It will be better to take a subway to go to Hualamphong. The subway will be better for people with small group and taxi will be more proper transportation for large group of people. Taxi is more convenient but you might get a stress because the taxi driver might take a round about with no reason. There are a lot of tourists who had a bad memory in Bangkok because of the taxi.
1) You can take a bus at the northern bus terminal to come to Nongkai or Udonthani. It costs about 500 baht for each seat. I recommend you to take a bus number 999 for a long ride. It costs about 800 baht and it will be very convenient. They provide a better bed than traveler bus. They also have a guide in the bus.
2) Traveler bus
You can take a traveler bus at Khaosan. There are a lot of tourists at Khaosan and travel agencies run bus which it goes to Vientiane directly. Taking a traveler bus at Khaosan costs about 700 baht. The good thing about this bus is that it provides connection with other lines. You can go to Vientiane by taking a bus coming from Nongkai. There are buses just go straight to Vientiane sometimes. They don't provide breakfast and dinner. They just stop at the rest area around 12 at midnight for 30 minutes. But if you are going to Bangkok from Vientiane, they do provide a meal. So the ticket price in Vientiane is little bit higher. Even though they called the bus VIP, the seat is very small and inconvenient. Your leg will be asleep because you will twist and turn for all night. It is very good to come to Vientiane directly and the bus usually arrives in Vientiane at 11 o'clock. Even though bus arrives in Nonkai at 6:00 A.M in the morning, it takes quite some time for changing a bus. You also have to wait at the border. You can take a tuktuk or you can walk to go to your next destination.
4. How to go to Vientiane from Udonthani airport.
1) There is shuttle bus at the first floor of the airport. It costs 85 baht for go to downtown Udonthani and 200 baht for go to Laos border. You can go over the border with this shuttle bus. It takes about 45 minutes.
2) Taking a round-trip but at the bus stop in Udonthani will be depends on your arrival time at Udonthani. Check the number 5 post of the bulletin board for the bus schedule. It costs 85 baht to take a shuttle bus for go to Udonthani bus stop from the airport. You can also take a tuktuk but it is hard to find it and you have to negotiate with tuktuk driver for the price. 30 to 40 baht is well enough to take a tuktuk to go to Udonthani bus stop. There are both good thing and bad thing to take a round-trip bus to go Vientiane. The good thing is the bus only stops once to go to Vientiane and a bad thing is that it takes a lot of time to go to your destination because there are a lot of people taking this bus and you have to wait for every other people to pass the border of Thailand and Laos. It usually takes 2 hour at least. The bus sometimes stops for 1 hour at the border of Thailand.
5. How to go to Vientiane from Nongkai.
1) If you arrived at Nongkai bus stop by the bus.
It was a long ride whether you took a bus number 999 or VIP bus. It will be good to take a breakfast because the bus arrives in the morning. Get a tuktuk to go to the border when you finish with your breakfast. It won't take a lot of time. The price to ride a tuktuk is very fluid depend on how you negotiate with the tuktuk driver. It could be 40 baht to 100 baht.
2) If you arrived at Nongkai bus stop by the train.
Take a tuktuk one that is waiting for the customer. It could cost from 40 baht up to 100 baht depends on how you negotiate with the tuktuk driver. You can also walk to go to the border if you don't want to spend any money for taking a tuktuk. You can go to the border by come out to the big street then make a left turn at it. If you go for about 500m of this road, the border will be on your right side. The big street is about 400m away from the station. There are a lot of backpacker walk to go to the border. There are people choose to walk to go to the border because the tuktuk driver sets a ridiculous price. The tuktuk driver will comes to you while you are walking and give you a lower price. That is how to ride a tuktuk with the lower price if you want to take a tuktuk. Or you can take a tuktuk at the opposite side of the station where you can see restaurants. The price is cheaper at there to take a tuktuk. There are tuktuks at the parking lot also. These tuktuks are for the local people and they are cheaper. Most of them are going to downtown Nongkai and you can ride them with lower price by negotiate with the tuktuk driver. Make sure you give the money inside of the tuktuk as possible if you are riding a tuktuk. There are some tuktuk drivers take the money and run away.
6. The next thing to do when you arrive at the border of Nongkai.
When arrived at the border of Nongkai, you have to fill out and submit the departure card at the Thailand border. There will be bus stop right after you pass the border. This bus goes back and forth to border and it costs 20 baht to take this bus and it costs 5 baht more for late ride or the weekend. Taking this bus at Laos border costs 15 baht. You can walk to pass the border if you don't want to take a bus. You just need to go over the bridge. There are some people walks and go over the bridge but I don't recommend you to do it. When you arrived at the Laos border, you have to go through the entry procedure. You need to issue the visa if you don't have one and you need to submit entry card and pay the entry permit tax. You need to pay some money - it depends on your nationality - for a visa fee and you need to pay extra 1 dollar on weekend or non occupational hour. The picture costs 1 dollar. You can pay with any other foreign money but it is best to pay with USD. You will suffer a loss in an exchange money rate if you don't pay with USD. When you pass the Laos border, touts will crowd around you for you to take their tuktuk or van. It usually costs 200 baht to go to your destination but low the price as possible. You can just ignore them and go straight to bus station at two o'clock way if you are alone or group of two. This bus is heading to the city of Laos. This bus has a sign which it says the bus was presented by Japanese. There is no air conditioner in this bus and most of people in this bus are local people. The price to take this bus only costs 5,000 kip. Its destination is Thalot Sao. You can walk or take a tutuk to go to your final destination from Thalot Sao. You can also take a tuktuk to go to Thalat Sao if you don't want to take a bus. This tuktuk is for share so there are a lot of people and load. It only costs 50 baht. You can go straight to the hotel or your destination if you negotiate well with the driver. They either ask for the extra money or not depend on your negotiation. They sometimes pass Samsenthai (One way road) and go toward the airport. You can get off at the appropriate place on this road and it is very close from Mekong Riverside. It is much better than walk from Thalat Sao. Most of tourists get their lodging near Mekong Riverside but it is not easy get one. There are many tuktuk drivers ask for your destination when you arrived at Thalat Sao. Make sure to negotiate with them for lower price. It will be helpful if you act like you are not going to ride their tuktuk. Most of local people negotiate and pay under 5,000 kip for tuktuk to go near Thalat Sao
There will be no problem if you get a train ticket that you want at the hualamhpong station in Bangkok. If you can not get a ticket, you need to get out of the train station as quick as possible. There are a lot of touts at the station and they make you to go to their travel agency at upstairs. Most of their tickets are very expensive. If you reserve for traveler bus from them, it is whole lot expensive than reserve from Khaosan. Plane ticket is also more expensive. Please do not follow them. If you can not get a ticket that you want, you can go to any of travel agencies at Khaosan and reserve for bus ticket. Or you can also go to bus terminal to reserve the bus ticket. You can get a train ticket from the travel agency at Khaosan. It costs 50 or 100 baht more than regular ticket. But it is not that expensive than you think when consider time and transportation expense. There are a lot of touts at hualamphong station.Or use http://www.thairailwayticket.com/default.aspx web site to make a reservation.