Vettel suffered in FP1 in Friday, only managing the nineteenth fastest time in the opening weekend's practice.
"Well the car is difficult to drive, but as I said, it's not the first time," Vettel told Autosport.
"So where are we lacking? I think we're losing down the straights, which is expected.
"We are lacking grip on corner entry with pretty much all four wheels. The car is sliding quite a lot. And. it's a handful.
"But yeah, it's very difficult to get everything right to get the lap together. But we're trying to make it a bit better for tomorrow."
Teammate Leclerc faired slightly better on Friday, finishing the first practice session in twelfth before breaking into the top ten with ninth in the afternoon session.
"I think we expected a bit worse on the low-fuel runs but then on the high fuel we expected a bit better," Leclerc explained.
"So we definitely need to try to find something for the race, otherwise it's going to be very, very difficult.
"We seem to struggle quite a lot with the balance and the overall degradation of the tyres during the high fuel run. So there's definitely a lot of work on that
"But I am quite positive to see our qualifying pace. It was very difficult to drive on the high fuel. That's why I was saying there's quite a bit of work, but I'm pretty sure we'll find a solution to try and fix this."
Bahrain International Circuit - Winter testing