The Thai immigration and customs building at the Chong Mek border crossing.
The Thai-Lao border crossing between Ubon Ratchathani and Pakse is a popular way to reach Southern Laos, or escape. Owing to limited flights between Bangkok and Pakse, this route is a cheaper and easier way. Since there are several budget airline flights available daily between Bangkok and Ubon, you can use this route at any time.
From Ubon, the actual border crossing is at a town called Chong Mek, avout a one-and-a-half to two hour drive from Ubon. Here, you pass through Thai immigration, and then walk across the border to Lao immigration. The easiest way to do this is through the international bus service that will take you all the way from Ubon to Pakse. Buses leave Ubon's main bus station at 7:00, 8:10 and 9:30 am. In all, it takes about three hours to get all the way to Pakse, where you'll be dropped at a small bus station just south of the city center. The fare, as of December 2011, was 200 Baht (5.19 USD).
From Pakse, the reverse trip back to to Vangtao and across to Chong Mek is available as well. The main difference is that you can book tickets at several travel agencies, and even some hotels, around town, and the price will include a free hotel pick-up to take you to the bus station. The cost in Pakse is usually around 80,000 Kip (9.60 USD).
Entering Laos, the path from the Thai immigration building to the Lao immigration counters is not well marked once you pass through the gate between the two countries. The Lao immigration building is about 100 meters down a dusty trail from the fence. If you're getting a visa-on-arrival, be sure to have the fees in US Dollars ready. The fee is between $30 and $35 depending on the country of your passport. You will pay extra if you arrive before 8:00 am, after 4:00 pm or on a weekend or holiday, or at any other time they deem fit.
Leaving Laos, be sure to keep back 10,000 Kip (1.20 USD) for the border fee. They will take Thai Baht, but at a considerably inflated exchange rate. Also remember that if you're using a visa-free entry to Thailand, you will only receive a 15-day entry for a land border crossing.