Vettel smashed the circuit record, but it is believed that the German was in a qualifying simulation with the very softest Pirelli 'hypersoft' tyre.
In contrast, Mercedes did not even order a single set of the softest tyres for testing.
Engineer Andy Shovlin said: "We wouldn't learn much about it in these temperatures."
World champion Hamilton said: "I don't know if that's a good time or not and I don't really care. It's testing.
"For us we've not been on low fuel yet so I have no idea where the car can go. I hope [Vettel's time] is good which means we have a race on our hands."
When asked if Mercedes has cured the car's 'diva' traits of last year, Hamilton answered: "Toto [Wolff] said that, not me. But he never drove the car!"
Vettel's time may actually have raised alarm bells at Maranello. Team boss Maurizio Arrivabene, for instance, reportedly looked stern as he walked into the Haas garage straight after.
Kevin Magnussen's time, set on a harder tyre, was not far off and it is believed that Arrivabene was enquiring about what fuel load he was on.
A GPS analysis done by Renault reportedly shows Mercedes with a clear, three-tenths advantage over Red Bull, with Ferrari trailing in third place.
When asked if he will be on pole in Australia, Vettel said on Thursday: "I don't know. Of course that's what we strive for, but it's not the time to decide that.
"Mercedes looked very strong last week and this week as well. They are in front. But you never know exactly what the others are doing."
Vettel did at least play down fears about engine trouble at Ferrari, after smoke was seen emerging from the red racer.
"We know about it and it's not a problem," Vettel said, "even though the photos are spectacular."
Also not willing to talk about potentially taking on Mercedes this year is Red Bull's Max Verstappen, although the Dutchman did say that he is "optimistic".
"The car is clearly better than last year," he said.
As for Vettel's laptime, Verstappen answered: "For sure it was a good time, but we expected that - just as we expect Mercedes to lead. We just hope to be as close as possible."
The Australian Grand Prix gets the 2018 season underway on March 25.