It will now run from 18-21 November and the F1 season will start in Bahrain on 28 March.
The Australian race, initially set to open the season on March 21, has been rescheduled to November 21 with F1 organisers citing "the ongoing situation regarding Covid-19".
The decisions on both countries come as a result of strict Covid-19 travel restrictions.
The season will instead start with the Bahrain Grand Prix less than four months after the Sakhir venue held two races as part of the 2020 season.
The third slot on 2 May is expected to be filled by the Portimao circuit in Portugal.
"Discussions with the promoter and authorities in China are ongoing with the potential to reschedule the race later in the season if possible".
Plans for the first Vietnam GP were axed late a year ago for political reasons, leaving a gap in the schedule that the Portimão circuit in Portugal - which hosted its first F1 race in 2020 - is tipped to fill. F1 said in a statement.
"While fans are used to seeing the cars on track on the streets of Albert Park in March of each year, we now have the honour of hosting an event at the business end of the season where the championship fight could well and truly be decided".
There is no obvious back-up date in the calendar.
A race at Imola in Italy has been added on 18 April and will be the second event of a record 23-race season.
Australia's new November date of means the campaign will be extended by one week, with Abu Dhabi Grand Prix closing out the season on December 12.
The majority of those races were staged behind closed doors but it is hoped fans could be present for the opening rubber in Bahrain and the following rounds.
"This is the right decision and we thank Formula 1 management for their co-operation in setting a new schedule", he said.
Swiss newspaper Blick reported earlier this week that before the start of the season, Formula 1 wanted every F1 staff member and team to be vaccinated against coronavirus.
F1 2021 start date: When does the new season begin?
Now, with Melbourne and Shanghai set to be postponed in 2021, Monaco likely to follow suit and the scheduled 25 April race date still vacant, it emerges that Formula 1 is once again putting together a "plan B" calendar. There was a compressed 17-race schedule through December with all races in Europe or the Middle East.