Ferrari driver Vettel said that there are "young drivers" who "deserve the chance" more than the returning Kubica.
Kubica's F1 career was brought to an abrupt pause due to a 2011 rally car crash, but the Pole says he is not offended by the comments as he prepares to tackle the '19 season with his new team, Williams.
"Everyone has their own opinion," he told Polish radio station, RMF FM. "Not everyone has to be happy that I am coming back. Certainly when it comes to other drivers, it's normal that the competition is big.
"You also can't really say that Vettel is a lot younger than me. On the other hand, there are a lot of young drivers who are entering Formula One - my team-mate (George Russell) for example."