Lewis Hamilton has admitted that he is not the favourite for the 2018 title.
"For now the car is much more difficult to setup and drive," Hamilton is quoted as saying by France's Auto Hebdo.
"It's not a car that you just put on the track and it works, like the Ferrari."
Like Marko, Red Bull boss Christian Horner was central to Vettel's development, with the energy drink marque having taken the German to his four titles.
"We are seeing an extremely mature Vettel," Horner told Auto Bild.
"We saw it with how he dealt with Max [Verstappen]. I think he saw the young Sebastian in front of him in China, because in his early years Seb was not flawless.
"But even more than that, Seb sees now that he has a car that's on par with the Mercedes. That means he can win on his own without driving and acting like hell.
"That's probably why it's easier for him to deal with defeats."
Next up on the calendar is the Spanish Grand Prix this Sunday.