I have not seen Vettel work in a struggling car, trying to get the most out of a car which can't win. Now he's clearly very tallented and has been well suited to the cars that Red Bull have provided him with over the last few years (excepting 2014, of course). But those cars have been winners.
Meanwhile you can't claim that Ferrari's offerings have been championship-winners-in-waiting. The engine in 2014 was a disappointment, the F138 looked gorgeous but couldn't deliver. A couple of cars have taken the championship to the end, but we can all agree that this was Alonso and not the car that was the foundation.
Alonso never looks like he's just circulating, he's always got his head down and his turn-in sharp.
There was a meme circulating around the beginning of the year which showed Vettel sitting in the 2012 car, and he had this look on his face like Alonso almost beat me in this piece of shit?! Surely taken out of context, but funny to think about. But probably not that far from the truth.
All in all, this driver swap is a downgrade for Ferrari.