However, Stewart is adamant that the Red Bull driver's car is the reason for Vettel's success.
"If you've got a superior car, it's relatively easy to win the championship," said Stewart.
"To really show you're one of the greatest, you have to deliver when you haven't got the best machinery.
"I have the highest respect for Sebastian, but he could not have achieved what he has without the [Red Bull design chief] Adrian Newey factor."
Stewart won the world championship three times.