Vettel is too immature and full of his self. Ferrari comes first at ferrari not the drivers.
I'd say by the time he is 27 or 28 he may be eligible. I just can't see him there before Hamilton, and when Hamilton is there, he may be reluctant to join.
Looking at the kinds of drivers that join Ferrari, Vettel is actually a very risky proposition in terms of his maturity.
Massa was a very mature and professional guy when he joined. Kimi was definitely level headed though he was very quiet.
These 2 guys were champion material before they joined or either was driving a ferrari powered car for years. I always remember for Felipe joined, he would always speak of driving the real deal, not just a car with the ferarri engine. He would say this repeatedly. Maybe this had to with him joining as well; a driver completely devoted to the prancing horse.
Kimi was a proven race winner and WDC contender. He was also, IMO, very threatening to shumacher, who was the best at the time. His choice was a no brainer.
Vettel now... he's just very fast. He's not really a threatening driver, doesn't have much experience or learn from his previous mistakes. Doesn't show much mastery or presence on the track like a raikonen or shumacher.
All in all he is not a proven driver as yet to sit in a red car. I'd say Hamilton gets there in 4 years. Vettel, maybe 5, or maybe he'll be at Mclaren before ferrari?