you can head to the mitsui outlet park which is nearby the airport ?
I dont recommend for you to go to downtown KL as you will have very little time left after allocating time for transportation and check in time.
Due to the Mitsui Outlet Park not opening until 10am it may not be a good idea to head out there as you need to check in 2 hours before your international flight.
Coming off a long red-eye flight I'd suggest using the Tune KLIA2 hotel which offers a day-use room from 7am-7pm for a rate discounted from their full day rate. A cheaper option is to book a 6-hour stay at the Capsule hotel at KLIA2. Both of these hotels are attached to the KLIA2 airport where your AirAsia flight to Vietnam will depart from.
Thank you very much for your advice
As well as Gordons suggestions, of also suggest the Sama Sama express hotel which is located in the transit section of the hotel, so no need to go thru immigration/passport control of your flying on a fly thru.
Just be careful of you book the tune hotel as its landside of Klia2, so you will need to go thru immigration.
Not sure about the capsule hotel thouEdited: 12 June 2018, 08:03
If you are booking transit hotel, make sure to book correct Sama Sama Express Hotel at KLIA2. Note that there are transit lounges which you can pay and use for 3 hours.