E sim and charger

    esims are only an option on some high end phones. My newer Samsung phones will work with one, but my older ones will not.
If your phone won't, simply buy a physical SIM card.
Chargers aren't much of a problem as you can buy adaptors that will allow your Australian plug to fit Vietnamese sockets. Mains voltage and frequency are the same in both countries.
If you have more than one charger, pack a small multiblock so one adaptor will do the job.

I've stopped buying adapters and instead choose the power strips that have multiple function female connections that handle multiple options including that previous image:

When shopping around (Lazada, Lotte Mart, Shopee, Mega Market, etc) there are all types of options available, from a simple setup with one multifunction input to construction grade cords.