The 1996 world champion says that Belgian Vandoorne's talent is undisputed, but he might need to shift into a higher gear to be successful in F1.
"I've known Stoffel since the beginning. He was in the same category as my son and I always admired his talent," he told Belgian broadcaster RTBF.
"He has a lot of potential. But in Formula 1, to be successful, you need something absolutely incredible, and with him we haven't seen that yet.
"He hasn't yet managed to get people to say 'Wow'. I think he has to let go a little, get rid of the 'nice boy' side and maybe become a little meaner."
Vandoorne has picked up eight points so far in 2018.