Analysis: Why the FIA have given Sebastian Vettel the green light over his overtake

Controversy currently surrounds Sebastian Vettel's third world title amidst claims that he illegally passed Jean-Eric Vergne's car in a yellow-flag zone on lap four of Sunday's Brazilian Grand Prix.

Ferrari have written to the FIA, the sport's governing body, seeking clarification about the legality of the overtake. However, it's understood that the FIA are content that the move was legal.
Here's the situation explained and how the drama unfolded...
Why are Ferrari seeking clarification from the FIA?
Because since Sunday's race, forensic examination of the on-board footage from Vettel's car, which was broadcast by Sky Sports F1's Race Control during the grand prix, has generated a considerable amount of online debate amid suspicion that the Red Bull driver overtook Vergne under yellow flags.

Had Vettel committed any such offence then it could, if he was to be found guilty, trigger a twenty-second time penalty to be added on to his result, demoting him to eighth and crowning Alonso World Champion in his place.
Quite the big deal then.
What do the FIA say?
Although Ferrari are yet to receive an official response, the governing body has briefed journalists that Red Bull have "no case to answer" as Vettel had passed a green flag to reconvene racing before he overtook the Toro Rosso mid-way down the backstraight.
What green flag?
It is far from obvious, not least because the flag and marshal are partially obscured on the footage by the throttle map graphic from Vettel's RB8, but as the Toro Rosso and Red Bull pass the Interlagos pit exit there is a steward positioned on a green-coloured gantry to the left of the race track waving a green flag. The more times you watch the footage, the more visible he becomes.
The track would have gone 'green' the moment Vettel passed the flag, meaning that his overtake was legal.
But why was his dashboard light still yellow at the time of the move?The dashboard warning light features in all F1 cockpits, officially known as a GPS Marshalling System, to make it more obvious for drivers the relevant colour flag waving as they go through the affected zone.
However, the light is only there to help the drivers as a way of a precaution and is not the be-all-and-end all in terms of signalling and only extinguishes when the car passes a green light. However, a prior green flag via a marshal, as in this case, takes precedence.
So just why was the light yellow at the exit of Turn Three?
Ahh. We admit, we're a bit stumped by this. We've reviewed the footage and found that the light was green when Vettel passed Pic on lap three, presumably because Pastor Maldonado had left the track further back. Yet the light had turned yellow one lap later. Why? We're not sure, although it's certainly possible that debris was on the track.
Did Vettel use his KERS during the yellow flag zone?
Again, we've reviewed the evidence, and the German only used the first burst of his KERS boost once passed the now established green flag. Prior to then, the on-screen graphic showed he still had his full allocation for the lap. In any case, there is no penalty for using a burst of the recycled energy in a yellow flag zone.
http://www1.skysports.com/formula-1/news/12433/8293852/Did-Sebastian-Vettel-overtake-Jean-Eric-Vergne-illegally-during-the-Brazilian-Grand-Prix-