The Pole is believed to be on the cusp of securing his return to the sport with Williams, years after almost severing his arm in a rallying crash.
"I think it's very tragic what happened back then," German Vettel told Sonntagsblick newspaper.
"He was regarded as a future champion. But I do not understand why he is pushing for a comeback now.
"Why did he not do this before?"
"For him it would be a nice story, but on the other hand it would be unfortunate for younger drivers who would also like that cockpit."
Nico Hulkenberg agrees that Kubica's return would be a "huge story" for F1.
"In testing you can see that the pace is there," he told Sport1.
"But you cannot simulate the racing action of the first lap in Monaco or Singapore.
"I hope he is able to race at the highest level, but we will only see that when he's in the car next year. I estimate the chances of his comeback at 50-50."
Kubica's last grand prix came in 2010.