Does anyone have any recommendations/experience of flying reliably between Siem Reap and Luang Prabang? Is Vietnam Airlines the best bet or is Lao Air OK? Do they really have traffic rights?

From what I remember- Lao and VN-air used to have strong cooperation and were in fact all done by VN-air, with some codeshare seats booked via lao. Thus superfluous.

Its by now some time ago I had to handle that, and esp. in Camb airlines come and go at the whim of the moment, so preplanning long ahead, as so many farang tend to do, will only lead to mishaps. It used to be a VN-737 doing Hanoi-VTE-SR-HCMC and back, I cannot recall direct flights to/fro Lpr, that would mean transit (and entry into Laos) in VTE.

ALT: AirAsia (IF you can stand it) via DMK, most likely the cheapest combo anyway. Also I dont know if international transit is now possible at DMK (again-as it used to be)