I finally made contact with the VN office in Sydney and they informed me that the transit service has been suspended (since the start of the pandemic) and remains so with no indication at this point in time of whether it will return.
I really don't fancy being stuck at SGN for 14 hours, so will look at getting a room at the Ibis Saigon Airport for the day. The only additional cost would be for entry visas, if you're not eligible for free visa.