Vettel ended the second practice session in third place, over eight-tenths down on Lewis Hamilton, who set the pace in his Silver Arrow.
"Hungary is the comparison, but today maybe is not the best day to compare," Vettel said. "Tomorrow will be more straightforward. On our side, I don't think it has been a clean Friday.
"We'll see, but I think Mercedes looks very fast and faster than us. There is probably more from us, but I would say that Mercedes left with the strongest impression today.
"I think we can still improve, the car is not yet doing what I want. I think in terms of balance we can improve. All the bits that we brought seem to work so that's positive news."
Speaking about the tyres, Vettel says the race is expected to be a one-stop, but noted that the medium tyre didn't hold up very well during the opening day of running.
"The soft is the tyre that we don't use so much. Obviously, it has great grip but it doesn't last so long, so let's see.
"Today it wasn't that bad, the medium tyre actually wasn't much better so we'll see tomorrow. Tomorrow is all on the softs and then the race I guess we will all try to survive with one-stop and try to manage the race."
Bahrain International Circuit - Winter testing