I don't think Newey would go to Merc to be honest. I mean, you're talking of shaking up a team's technical structure, that has recently been shaken up anyways? I don't think so... Though I could be wrong.
The draw of Mercedes and returning it to winning ways is prestige in its own right.
Red Bull coming into F1 and winning is all very well, but it bears no semblance of the legacy Mercedes had/have and resurrecting it.
Newey is already immortalised in F1, but that would be the cherry on the cake.
And this is Newey we are talking about. Any team on the grid would fit him into their technical structure.
I really do see history repeating itself with Vettel though. Ferrari are 2nd to Mercedes because Alonso resides as the No.1 and the apple cart would be well and truly upset if Vettel moves to Ferrari. It is possible hence why they are a close 2nd to Merc in landing Seb, but I think it unlikely Ferrari will risk Alonso leaving.
Mercedes would bend over backwards in accommodating Vettel, and as a potential 4 time champion by 2014 I reckon he would want a new challenge come the turbo era.
The caveat to all this is that Mercedes show steady signs of progress between now and then.