For Landing visa

Hi^^
Our schedule is Incheon Ho Chi Minh Sydney Melbourne imcheon.
But I have heard that listening to Ho Chi Minh City re-entry visa is required.
In this case you need an invitation?
Is there no invitation to get a visa on arrival?
Now I have a visa-free entry to Ho Chi Minh City
Tomorrow will depart Sydney
If you apply for a letter of invitation issued by How long
Plz. Help me. I need your help.