I've asked before, nobody answered, why did Alonso ever leave Renault in the first place?
And then: what makes Vettel imune to the same temptations?
With the engine regs changing or not, depends what happens, there could be implications all around. If they do change, as with the Lewis train of thought and what has been described by the media as potentially shrewd, it is thought that the manufacturers will have an edge as they have more time to develop the car around the new engine packages. Hence Ferrari would be quite a safe bet. True, Red Bull are pretty much the Renault works team, but it isn't quite the same.
If the engines are delayed, Renault may quit and Red Bull are left up the creek.
Seb has an exit clause for 2014.
The paddock gossip say he has signed. This type of paddock gossip is similar to the signing of Alonso, Kubica and Kimi. Ferrari work on long term pre-contracts with star drivers. It ties in with the Ferrari way of doing things.
The fact that Perez wasn't signed due to "inexperience" even though that is unlikely to be the case as it is believed that Hulkenberg is in with a shout of the 2013 seat (sounds like any driver for 2013 is to be short term), suggests that someone is already lined up for 2014. To plan that far ahead suggests that Seb is the obvious candidate.