We flew into Vientiane and out of LP, although the latter entailed an 8 hour stopover in BK before catching the red eye special back.
You can fly from Vientiane to LP, I think 3 flights a day, but even though the minibus ride to LP is around 5 hours on the packed public mini buses it goes through some impressive countryside that wouldn’t otherwise be seen, and the trip from LP to NK is even more spectacular. The roads were made the whole way, if a bit potholed in parts, but I know they get rougher if you head further north - a friend of ours who did this complained people kept throwing up around her.
On the way back from NK to LP we tagged along with 4 Kiwis who had hired a private mini bus which had a sane driver who happily stopped for photo ops and dunny breaks, air con and operational seat belts- did it in under 3 hours and dropped us at our hotel. If we do it again we’ll definitely go with the private mini bus idea. Still pretty cheap by our standards-e.g. 100 US split 6 ways for a 12 seater for the 3 hour trip from NK to LP.
River transport is a great idea but it ain’t as easy as it used to be thanks to the proliferation of Chinese financed dams in Laos, I think around 7 are either in place or in construction on the Mekong in Laos alone, much to the annoyance of Vietnam.