That would be a very long and uncomfortable bus ride. Perhaps better to book from Vientiane to Pakse - take a couple nights in that area if you have time - then go onto PP.
Around Pakse you can easily relax at least for one night in that town before heading on, or go down to Champasake for two nights and see Wat Phu, or go down to Si Phon Don (4000 Islands) for 2/3/4/7/12 nights depending on how you like it and head on directly from there through any of the many tickets offered around the islands.
true as Casey says here.
sleeper bus to Pakse is your best option (10h and mostly arriving 6 in the morning). go straight from Pakse to Si Phan Don and relax a night and move on the next day to Phnom Penh .
Other option is when you arrive in Paksé in the morning catch a ride to the border and hitch hike after the border to Stung Treng in Cambodia and stay the night there (that's what I have done but coming from Si Phan Don though). The ride from Stung Treng to Phnom Penh should normally take about 7 hours but our bus broke down (keep in mind those things happen all the time) so it took us between 10-12 hours I guess.
A few options to go for but definitely have to split it up with an extra night somewhere!