The Ferrari ace has questioned the timing of Kubica's possible return to the sport - he has been out since a rally car crash in 2011 left him with life-threatening injuries and almost cost him the use of his right arm - and, at the age of 33, means there will be one less place for a younger driver, according to Vettel.
"I think it's very tragic what happened to him back then, he was considered a future champion," Vettel told Swiss newspaper Blick.
"However, I do not understand why he is now pushing for a comeback. Why did he not do this earlier?
"For him it would certainly be a nice story, but on the other hand, it would be a pity for younger drivers, from whom he would take away a race seat."
Getting a drive in Formula One is almost impossible and if Kubica does return it would mean just one new driver for the next season with Sauber confirming Charles Leclerc as part of its Alfa Romeo deal.