Vettels move has ended Jenson Buttons career, I doubt that he is interested in driving for a such as Sauber and that Alonso will replace Magnussen, after all he is the better option for the future.
It would also be silly to dissolve Raikkonens contract to keep Alonso, that would mean they would have to pay him off again.
Is Alonso taking Santander with him or will they continue to sponsor Ferrari to 2016?
I can understand Vettels decision. It started with Newey, then his race engineer who is scheduled to be replaced next year and a couple of days ago his chief mechanic and Red Bull parted ways, plus the numerous mechanical problems particularly at the beginning of the season.