You mean two separate tickets, each with a unique booking number? I think that the AirAsia intl-intl flight connection service, AKA:Fly-Thru, requires both segments be issued under one, single booking number...
7. I have already booked 2 different flights and noticed that I can opt for FLY-THRU service with a promotional fare. Can I convert my bookings?
No. This service is only available when flights are purchased under one booking number.
https://support.airasia.com/s/article/Does-AirAsia-provide-stop-over-en?language=en_GB
So I'm not certain you'll be able to remain in the international transfer area? Maybe look for an FD rep on exiting the the first flight and ask them about your options?
I based my opinion on three+ year old information, pre-COVID. Things may be different now.
If you are forced to arriving immigration, and are denied entry, I'm not sure 3 hours will be enough to "negotiate" your way onto the DMK-SGN/HAN flight? I assume you have the proper visa for Vietnam.
You could reissue the ticket under one booking and thus qualify for Fly-Thru, but this will require you to pay additional money. Maybe arrive a bit early at PNH and ask FD about some options?
Is there a reason you purchased two separate tickets?