So it was interesting that at the recent Kitzbuhel skiing world cup event, Vettel - who won all four of his titles for Red Bull - was photographed with the energy drink outfit's current F1 drivers.
Marko, however, played down speculation of a return for the 32-year-old German.
"With Max's extension, I can't imagine the two Vs being with us," the top Red Bull official, referring to Vettel and Max Verstappen, told Germany's Motorsport-Magazin.com.
"Financially alone it would be very difficult."
However, Marko said Vettel is still a top F1 driver. "If Sebastian feels comfortable - if he has a car that suits his driving style - then he is absolutely there."
He thinks one of Vettel's main disadvantages at Ferrari is political.
"Sebastian is a very straightforward athlete who doesn't want to know much about politics and doesn't want to fight," Marko said.
"Leclerc has an advantage with his manager (Nicolas) Todt, who is certainly very good in all of those areas. But he (Vettel) can counter with his performance."