But Hamilton's Mercedes team-mate Valtteri Bottas countered: "As a team we've learned a lot from the package we had in testing and how we can optimise it better for the first race".
Brawn was the technical director at Ferrari between 1997 and 2006, the team's most successful period with six consecutive constructors' championships and five drivers' titles for Michael Schumacher.
However, Rosberg admits that Vettel needs to drive a "perfect season" from the cockpit, and fend off the challenge of his new teammate Charles Leclerc.
Scuderia Ferrari has unveiled the revised livery that it will run at this weekend's season-opening Australian Grand Prix.
"What we see from testing, we think [we are underdogs]", Bottas told reporters in Melbourne. "We're going to be kind of chasing".
It seems like they [Ferrari] had a few good tenths [advantage], but here it is such a different track. It's the same thing with the training; we try again to improve from previous year. We think they're going to be strong because Honda looks strong now as well and they've always had a good vehicle. [And] I think it's a case of adding pure performance, but even then we know we haven't unlocked the full potential of the auto.
"It's always nice to start here in Australia but it will be a very long and tough season, so just being focused".
The cars feature new front and rear wings, aimed at making it easier for them to get closer and overtake, while the driver that sets the fastest lap can earn an additional championship point, so long as he finishes in the top 10 in Sunday's race.
"The name of this year's vehicle, the SF90, is already a celebration of this anniversary and, on top of that (we have) made a decision to honour this achievement in an even more evident manner in Melbourne", Ferrari said in a statement.
Max Verstappen, who at 17 was the youngest driver ever to start a race, is now a 21-year-old "veteran" in his fifth season and the main man at Red Bull, who also have 23-year-old Frenchman Pierre Gasly racing for them. We have experience, it's no coincidence that we are world champions so we have to be diligent and stay balanced.' He added: 'But as a driver for sure I have to figure out how can I pull more out, which I don't have the current answer for'. 'We had our weaknesses last year here and there and we obviously try to fix those and do much better this year. Over the past two years, the way they've been able to improve the vehicle during the year has been pretty rapid, so for sure they're going to be a threat as well.