For nightlife, I can recommend the riverside along Street 11, approx. between corner St. 46 and southern corner St. 10. Many little restaurants, bars and foodstalls there.
Definitively open after 10pm is, for example, "Lotty's" (near corner St. 10). They close at the outside at 11pm and at the disco/karaoke bar inside (downstairs) at 12pm.
I was in Pakse for a week and stayed at the Champasak Palace hotel, a little bit away from the centre. The town seemed to close at 10pm at night even the hotel bar closing up. I couldnt find anywhere that stayed open past then. As I am going back I would be happy if people can give me a hint on a bar etc that stays open at least to midnight!