I traveled from Pakse to Vat Phou by motorbike. The countryside was very beautiful and interesting... The villages we passed looked like people were living as they had been living for hundreds of years.
I was fortunate to meet an archeologist in the high complex who was kind enough to show me around and explain the significance of the site. He said this was where the Khmer kings came to be "cleansed" before beginning their reign. So I imagined I was there as a preparation for the big exploration in Cambodia...
Vat Phou itself is small and demure compared to the majesty of the Khmer cities in Cambodia so I recommend seeing Vat Phou first as kind of a preview of things to come in Cambodia.