2017-05-28 15:38:39 UTC
1) Vettel had had better long run pace in FP.
2) Raikkonen was leading the race and got priority
3) Vettel stayed out and had very good pace
4) Vettel got lucky when Sauber pulled in Ericsson just as Vettel was going to come up on his rear - this creates some open air for Vettel to run in
5) Vettel gets in the fast laps and makes just enough of a gap to pit and come out in front.
I don't think anything was planned ahead of time. It is possible that Ferrari saw Vettel's pace and decided to give him a chance to make the overcut work. They couldn't know that the Sauber would pit - indeed initially it looked like Vettel was going to run into traffic and Raikkonen would thus end up in front on the row.
Whatever, Vettel had to make it work. And he did. He was faster.
Interestingly, and again Ferrari could not have known this in real time, the undercut didn't work in this race, and the overcut worked nearly every time.