The four-time champion cited the Silver Arrows’ strong record in Sochi - they have secured every pole and led every lap since the inaugural race in 2014 - and also pointed to previous race weekends as proof that form can fluctuate wildly as teams trial different settings.
"I think Mercedes will be fine; it's a circuit that suits them, so they will be strong tomorrow," said Vettel after topping FP2 with a 1m 34.120s - the fastest time ever set in Sochi, and 0.67s quicker than Bottas in third.
"I don't want to make this personal but I think last year people expected Williams to be the fastest after Friday if I remember right, and obviously it turned out Mercedes were.
"That's how sometimes you can be misled. I think there are a lot of things we can play with in the car, loads, engines modes… this track especially there are a lot of things you can show or not show.
"I think the most important [thing] is that we talk about ourselves, our balance - and I think we improved throughout the session so I’m reasonably happy."
Mercedes chief Toto Wolff though said his team had work to do, having apparently trailed Ferrari on both single-lap and long-run pace.
"Ferrari are extremely quick on one lap - and their long run was impressive," he said following FP2.
"We've said before that  is all new - we've been good on track for the last few years but the cars are totally different now so you cannot anticipate that it will all just continue.
"We have Fridays sometimes when everybody does weird programmes to find out where they are... so here we go. Work to do."
Ferrari have won two of the three opening races in 2017, with Vettel currently leading Hamilton by 68 points to 61 at the top of the championship standings.