Im looking to fly from Hanoi to Luang Prabang at the begining of June.
Flights look to be around Â£100 at the moment, but i dont want to book in advance in case plans change. Are the flights likely to be more expensive nearer the time??
Prices can go up nearer the time depending upon availability. Have you checked the T&Cs on the flight you've found? It might allow for changes or cancellations, even if for a small charge, in which case it could be worth booking now if your plans are 80% fixed.
Lao Airlines is a couple of dollars cheaper using their website, and I understand their service (they serve a meal) is very good. Either VN or Lao is fine, and Lao codeshares on VN. (If the aircraft is ATR, it's Lao)