The BBC does mention a green flag at the Marshal post right after the pit exit:
” However, on the footage from Vettel’s car it is not possible to see clearly the marshals’ post that is situated on the inside of the track at the exit of the pit lane – after the yellow light at Turn Three.
It is possible that the marshal there was waving a green flag. If he was, the pass on Vergne would be legal and the result would stand.
A green flag is waved at that post as Vettel passes the Marussia of Charles Pic on lap three, but it is not clear whether it was still being waved on lap four. ”
The in-car footage appears to show the green still waving on lap 4. I’m sure the FIA will have the footage from the far end of the straight that will show it clearly — it’s very clear on the broadcast the marshal swapping out the yellow for the green before Vettel comes by. On lap 5 you can also see that the flashing light at the exit of turn 3 has been turned to flashing green during the race (as the leaders went through).