I've been following this very keenly, and it is turning out to be an interesting story.
The only thing official at this moment in time is that Vettel is leaving RBR. It is widely believed, and Christian Horner and Niki Lauda have confirmed, that he has signed with Ferrari, but there's no official word yet.
Basically, Raikonnen and Alonso had contracts for next year with Ferrari. But with Honda returning to the sport next year as a partner to McLaren, they wanted to sign Alonso. The rumors are that Alonso agreed with Ferrari to terminate his contract for next year so he could sign with McLaren-Honda. Ferrari, understanding that he was in talks with McLaren-Honda, quickly grabbed Vettel to fill that seat.
Alonso was using his contract with Ferrari to get more money from Honda. But the Vettel announcement really threw the cat amongst the pigeons, because Alonso lost that leverage. So Honda offered less money, and Alonso didn't sign off the Ferrari contract. So essentially, Ferrari has 3 drivers employed for next year, which leaves Alonso with 2 options at this moment -
a) sign-off his contract with Ferrari and sign with McLaren-Honda, another team, or even take a sabbatical. OR
b) make Ferrari terminate his contract themselves so could receive $30million (the worth of his remaining contract) and then go off to McLaren-Honda or whatever.
So, in either cases he will be getting less salary from McLaren-Honda then he might have. But Alonso being Alonso, usually gets what he wants. We might hear some concrete news this weekend.