Hamilton cruised to his sixth F1 Championship last year after opening up an 88-point gap over Vettel.
However, Ferrari have looked extremely dominant during the first seven days of practice in Barcelona.
Vettel and new team-mate Leclerc have been almost untouchable and they are sharing secrets to try and break Mercedes’ five-year stronghold on the Championship titles.
The German, who is out for his fifth world title title this season, was asked if he was advising his young compatriot Leclerc.
“I’m asking him,” Vettel replied.
“We sit right opposite each other and we share the same seat in the car so we are talking quite a lot, about the car’s behaviour, his feedback, my feedback.
“We are listening to each other. Everybody surrounding the table is listening so as think as much exchanges as possible.
“I don’t recall every question he’s asked but we’ve talked about the car and our feelings about the front-end, the rear-end and so on.
“I think there were a lot of questions but not just from his side, also I’ve asked a lot of questions.”
Vettel’s former team-mate Kimi Raikkonen joined Alfa Romeo for the upcoming season.
And the 31-year-old discussed the differences between his two allies.
“We are always talking about the same things and trying to find the best balance and pointing out the weaknesses of the car,” Vettel added.
“I think in this regard it’s been fairly similar. It’s probably a question I can’t answer.
“Outside it’s difficult to see but from the inside you can see it’s a bit different.
“Needless to say he’s fast and knows how to drive a car otherwise he wouldn’t be here.
“I always got along very well with Kimi. I think I have probably spoken more with Charles in the last week than with Kimi up until the Barcelona race!”