Raikkonen, who clinched the Drivers' Championship title back in 2007, walked away from the sport at the end of the 2009 season after parting ways with Ferrari.
The Finn switched his focus to the World Rally Championship but was constantly dogged by rumours of a pending return to Formula One.
Now, after two years away, his comeback will be realised with the 32-year-old signing a two-year deal with the Lotus Renault GP.
However, Vettel reckons Raikkonen could have a tough time next season as, like Michael Schumacher did upon his return, there is a lot for him to adjust to.
"It's difficult to judge," Vettel told France's 20minutes.fr.
"We can look at Michael's comeback. He left when he was at the top but when he returned he said that much had changed and that he needed time to adjust.
"If I look at the car I drove two years ago, so many things have changed... It's amazing.
"Even during the season new buttons appear on wheel. I can imagine that it would be difficult to return straight away to the same level as before."
However, the reigning world champion believes F1 needs a character like Raikkonen in the pits.
"Sometimes we are criticised for not being interesting enough - that F1 lacks strong characters," he said.
"This is not entirely the media's fault, nor the drivers', it is a bit of both.
"Kimi is criticised for not speaking and behaving as he does. I agree sometimes some of his attitudes are perhaps not terrible. But on the other hand, it simply reflects who he is."