Bangkok to Siem Reap to Luang Prabang to Hanoi. Is this possible to do without booking flights?

I know Vietnam Airlines fly from REP-LPQ-HAN and I could easily get Bangkok Airways from BKK to REP, but it would be so much more of an adventure(!) if I could do it by bus or coach.

I will be travelling solo, and it will be during March next year.

I am backpacking so I aim to save money by not taking flights.

FWIW, i do not want to visit anywhere else in Laos or Cambodia apart from the cities mentioned (apart from a daytrip to the Plain of Jars).