Ferrari have been the dominant force in recent weeks, with Leclerc taking four pole positions in a row.
However, Valtteri Bottas led a pair of all-silver one-two scores in Friday's practice, with running now to be halted until Sunday morning in Suzuka due to Typhoon Hagibis.
With more than half a second left to make up to Bottas, Vettel is not yet feeling the pressure.
"Maybe the other cars are different, maybe ours is the same - it depends from which way you look at it," he said.
"For sure we saw today that Mercedes was very fast and feeling very comfortable straight away.
"Maybe they [Mercedes] did a step forward, maybe the track suits them more than [the others]. A lot of things might be the case. I think we can improve, though, so we see where we are on Sunday morning."
Both Vettel and Leclerc tried to make up ground late in FP2 by running extra qualifying sims when the track was rubbered in, but traffic stopped them improving significantly.
Asked how the Scuderia can improve, Vettel added: "Hopefully more with skill and commitment rather than data and computers.
"As I said, I think there was a bit of margin, it was not a perfect day for us, so there's a bit more in the car."
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