Book on the airline's website.
Use search tools to inform yourself of the usual fare then book when you see the fare reach that level.Edited: 07 August 2018, 19:32
I have flown with vietnam airlines & vietjet - book directly from their own websites below
Both i have found to be very good , and for the most part i have had no issues
Vietnam airlines would be classed a full service airline whereas Vietjet a budget , however for these short flights the only difference really would be you would need to purchase baggage allowance for Vietjet (not expensive) - you would also get a light meal with Vietnam airlines, but i would book the cheaper , assuming flights are at a suitable time.
Also would definitely book before you arrive, and both have calendars so you can play around and guess when would be the cheapest time to book - for example if you were booking vietnam airlines 10 months the prices tend to be more expensive , than waiting to nearer time
Alternative - not necessarily any/much cheaper is overnight train - done a few times with kids - not bad experience but would only recommend if you have loads of time
On the Vietnam Airlines site, select multiple destinations. Enter what flights you want. Not strictly return but if you are looking at just one ticket for all flights, near enough.
Vietjet is a point to point carrier (budget), I doubt if you'll be able to do the same on their website
I would also check how many flights are flown on the day(s) you are looking at. Some regional airports sometimes have only one or two flights per day to and from certain destinations. Those flights I would book before arrival.
Not sure booking a return is cheaper than booking 2 singles with either of these airlines, so no hassle booking 2 singles with different routes
The Hue - DaNang stretch on train is definitely worth doing as scenery is tremendous - try and book on left hand side of train, but from memory people pretty much sit where they want regardless of seat booking !
Not sure your dates , but vietjet have flights from Hanoi - Hue from mid September - December for 24 USD per person - there are 2 flights a day and this price is virtually any flight on any days in this time period
Once you select flights there is some additional costs (some optional - luggage / some compulsory - card charge) but would definitely be less than 70 GBP. Flights before this will be more expensive and flights for 2019 not yet out.
Vietnam airlines have flights from 750K dong which again is around 25 quid for certain dates (includes luggage)
Without knowing your dates , i cannot say for sure you will get cheaper than 70 quid , but if it is over 2 months away , i would say for sure you will be able to , and keep an eye out but typically booking 2 - 4 months before is optimal
Yes the internals will be booked in the new year. Cheers