When to buy flights to Vietnam/Cambodia?

[Moresaltedcaramel]

Planning a trip to Vietnam/Cambodia next year - dates are flexible, probably April or May. Does anyone know when the optimum time to buy tickets would be? Are they likely to go down? e.g. in January sale?
Any travel agents able to advise? Thank you!

I think the general rule is to either book a long time ahead, or just before you travel.

I've been to Vietnam and Cambodia but I traveled overland from Thailand.

I would expect return flights to be around 500 outside of the school holidays.

IME book as early as possible for the best prices. I am going to Vietnam in April (I live in China) and booked my flights in October.