Ferrari team-mates Felipe Massa and Alonso were second and third behind Vettel in practice one and sixth and eighth in the afternoon.
After a winter too close to call, Vettel's practice performance installed him as a clear favourite for Australian Grand Prix victory, which Alonso said he had anticipated.
"I'm not expecting any major surprises here, we already knew we were not the quickest and that was confirmed today," he said.
"The car responds well, but we know there is still much to do if we want to fight with the very best."
But Ferrari design chief Nikolas Tombazis insisted it was still too early to say if Red Bull was clearly ahead.
"In terms of performance, we are approximately where we were expecting compared to Barcelona," he said.
"All that applied in Barcelona applies here during Friday practice and we are unable to comment on our pecking order relative to our competitors.
"Our performance is as we expected but we need to wait a bit more for definitive comments."
AUTOSPORT Australian GP coverage:
FP1 FP2 Qualifying Race Gallery Live commentary Official quotes