Is passport 6 month validty rule from date of entry into Vietnam, or date of visa expiry?

I will travel to Vietnam at the beginning of February with my family for a 2 month stay (we do not have visas yet). My daughter's (Australian) passport will expire in the middle of August 2020. I want to check if this will be ok (her passport will have 6 months validity when we arrive, but only about 4 and a half months validity when we plan to leave Vietnam).

Thanks for your help.

Regards,
Daniel