Should still have enough time if not booked through on one itinerary, but if not so booked, the risk falls squarely on you.
1/ are you on single PNR or flights booked separately?
2/ are your Australian passport holder?
Vietnam Airlines is not a codeshare partner of SQ so I believe this is booked as 2 separate tickets. In this case, you will have to go through customs and check in again for your Vietnam airlines flight.
Also Vietnam Airlines departs from T4 whereas SQ will land at either T2 or T3 so you will have to make your way to T4 after clearing immigration.
I thought as much, many thanks for confirming
Yes you must go through customs, then go to taxi stand to get a Uber direct to Orchard Road.
Go shopping for 2hrs then return to airport to check-in for your flight again.