Mercedes impressed in last week's final Barcelona test, setting fastest times overall with single-lap efforts but also delivering strong long-run pace.
Although it is not clear yet just how much of a step forward Mercedes has made, Vettel believes that Hamilton and Rosberg could surprise.
"I finally took the chance to watch some of the other teams for a bit," said Vettel, reflecting on the conclusions from last week's test.
"Nico had a great final day of testing in his Mercedes, which shows how dangerous he and Lewis will be in the future. But there are a lot of others to keep in mind as well."
Vettel is pleased with Red Bull's own progress, but has drawn short of predicting that the outfit will be the pacesetter from Australia.
"All in all I am fairly happy with how everything went in these last weeks," he wrote on his own website.
"The team did a great job with the car and both Mark and I felt comfortable right away.
"We haven't come across any big problems and the smaller ones have been dealt with straight away. The RB9 has been so reliable that it's been a pleasure to drive.
"We learned a lot over the winter tests and we'll be bringing that new knowledge with us to Australia."
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