I am currently in LP, took the boat trip to Pak Ou cave and enjoyed the scenery. I have about 8 days in Laos before heading to Danang.
I am interested to see karsts. From what I read, the prettiest stretch is between Nong Khiaw and Muang khua. But others recommend Vang Vieng. The bus LP - VV is simple. But the trip to NK-MK requires an overnight in NK, then another overnight in MK, then overnight in LP before heading south or take a flight to Danang.
Today is my 5th day in LP and I don't mind spending more time in LP. Some recommend to skip VV.