The problem is that I don't think that this situation is explicitly covered by any rules. There are rules about what should happen on straights and coming in to the breaking zone for a corner. As for what happens at/just after the apex it's more philosophical than strict rules. It basically comes down to what is generally accepted by the drivers and stewards rather than specific rules. This seems to boil down to who gets control of the apex of the corner. That's why it wasn't punished; there will have been a strong enough argument that Max had control of the apex and the corner, so it was Vettel's responsibility to take avoiding action.Granted, same had happened before in the race too with Vettel pushing Alonso wide and gone unpunished, but that doesn't make it any more ok - they both should have been punished, or the damn rule needs to be removed so everyone can feel ok doing it.
At this point, I think that Vettel was always going to put himself in to a difficult situation with Max whenever he tried to overtook; I'd have been surprised if there wasn't a complaint from Vettel however Max managed to get past - Vettel is coming across as more of a grumpy old man than a racing driver when it comes to Max.