They key point is that if the marshal at point B was displaying a green flag on lap 4 when Vettel overtook the TR between points B and C then he will have already passed the first green flag or light and be out of the yellow flag zone, regardless of what his dash said, and overtaking is allowed. If however the marshal at point B was displaying a yellow flag then the first green flag or light that Vettel passed is at point C, after the overtake, and the overtake is illegal. Green flag, yellow flag zone ends at point B, no green flag, yellow flag zone ends at point C.
It is impossible to judge what is happening at that marshal point from the onboard video alone (belive me, I've tried
). However, there is video showing the marshal change from a yellow flag to a green flag at that post B on lap 3, the lap before the overtake. Also DRS is enabled on lap 3, and the straight between points B and C is the DRS zone, and the FIA disable DRS if its in a yellow flag zone. IIRC the caption comes up on the live TV feed to say DRS is enabled at almost the exact time the yellow flag is withdrawn and replaced with a green one. So it seems to suggest there may have been a green flag there on lap 4.
This is the point that needs clarification, and as yet there isn't anything concrete to say yes or no. However there does seem to be more evidence to suggest there was a green flag than there wasn't, for the time being.
I really hope the FIA clarify this unquestionably. A picture from a fan would be good