Vettel is something of an Australian Grand Prix specialist, having won the race three times.
Nico Hulkenberg will replace the four-time world champion.
Aston Martin are the third team to kick-off their season with a launch but their car is believed to be the closest so far to being the finished product.
Vettel had finished second in the August 1 race but Hamilton took the position after the German was ruled out for not having the required amount of fuel left in his car for samples to be taken.
The 33-year-old German replaces Mexican Sergio Perez at the team formerly known as Racing Point.
Ferrari drivers Charles Leclerc and Sebastian Vettel have been seen mixing with people outside their individual team groups.
The two-time Le Mans 24 Hours endurance winner and former Ferrari driver, who turns 39 this month, secured his championships in 2005 and 2006 and most recently raced for McLaren in 2018.
Australian Daniel Ricciardo will leave Renault and race for McLaren in 2021.
German media, who flagged up the departure on Monday night, reported Vettel had rejected the terms being offered by Ferrari.
Ferrari have said Vettel remains their first choice to partner young Monegasque Charles Leclerc but reports in Italy have suggested they want an answer by the end of April to ensure potential replacements are still available.