There is a daily bus from the main bus station in Korat (bus station #2 near Makro). It currently (2012) leaves at 9 am. Cost is 320 baht only.
The bus stops 10 minutes at Ban Sida for toilet.
Here’s the bus we took:
Later, you stop after Khon Kaen about 30 minutes for eating. You must pay your lunch, the fried rice with shrimp is excellent for 30 baht!
You also stop around Udon Thani 10 minutes for toilet.
Bus was almost empty, as you can see…before they packed it 10 minutes before the border. (see below)
Very near the border, the bus will stop to FULL IT COMPLETELY of Thai stuff to pass the border. Do not ask me what it is….mushrooms from China, toilet paper, it was fully packed. Then, you start again and arrive at the border.
Foreigners need a visa. If you do not have passport pictures, they will make 3 quickly for 50 baht.
Going out of Thailand:
And you leave Thailand. Do not forget your passport and the departure card. The bus waits for you on the other side. You go back inside. It takes 10 minutes.
After the bridge, you make your visa for Laos. Bring a pen or buy one for 10 baht. The visas depends of your country, Canada pays the most, 42$ US or 1,800 baht. I think Americans pay 35$. That takes maybe 20 minutes, and the bus wait for you after.
The bus goes directly to bus station in Vientiane. You arrive at about 4pm. Very comfortable. Air con. Thai music all the time (bring an Ipod or phone with headphone if you can support Look tung music!).
I really enjoyed that trip. Easy, nice, comfortable and 320 baht + visa in Laos. Vientiane is not the best city but food is good. We went later to Vang Vieng (4 hours in bus) and Luang Prabang (7 hours from Vientiane in Bus).
EDIT 2013: The schedule and price of the bus has changed. The bus is now leaving Korat at 11:30 each day. It is a VIP bus. You arrive in Vientiane around 8am to what I was told. Price is 540 baht. Someone I know will try in the next days.