Since I discovered the sleeper bus between Vientiane and Pakse I discarded to flight to the south, but I have a doubt: what's the difference between VIP and normal sleeper-bus accommodation from Vientiane to Pakse?
I was in SEA at the beginning of the year. Firstly, good on you for not flying south. It's a very expensive and unnecessary flight for a trip that can very easily be made on land.
When it comes to VIP buses in South East Asia, there isn't usually a big difference, but it does depend on the price. Sometimes a VIP bus may give you better seating, A/C, only other tourists/foreigners on the bus and/or it could be faster, as some of the normal sleepers pick up a lot of locals along the way who sit in the aisles.
However, that being said, the VIP buses can be exactly the same. With the trip from Vientiance to Pakse I would probably just get the normal sleeper-bus unless there is a major price difference, which, in that case, probably means the normal is full of locals and will be a less comfortable ride.