At the last Laos trip on the way back from Don Det to Thailand we did stop in Champasak to visit Vat Phou. If you did see Angkor Wat you do not have to do that – there is nothing more to see. But if you have the time – it makes a nice stop on the way and Champasak is nice to even spend a few days there!
The temple is over 1000 years old which makes it pre Angkor. The architecture is Khmer but the religion is Hindu. It did lost his importance when the Khmer moved the capital to Angkor but was never forgotten. Much later Vat Phu became an active Buddhist sanctuary with it is till today.
Since 1991 the whole complest is under renovation and there is a little museum at the entrance as well.
There are a lot of tourists doing that as a day trip from Pakse as well