This whole incident was super marginal. I don't think you can criticise the stewards for following the letter of the regulations and penalising Vettel for an unsafe reentry. He knew Lewis was behind him. He could have probably not gone straight back over to the racing line, and the sporting thing to do would have been to briefly slow and let Lewis pass. Who knows, he might have re-passed Lewis later in the lap.
From the comfort of my sofa, it looked like Vettel could have perhaps got out of the throttle after getting back onto track. Instead, he straightened up and got back on the power immediately because his racing instinct made him keep pushing for position. By doing so, he effectively squeezed Lewis right up to the wall. It was SO close to being a double wreck.
There's no clear reason why Vettel would have straightened up after correcting the oversteer and gone to the track edge - unless he was momentarily recovering from the surprise of what was almost a huge smash, and in that moment, forgot to leave enough space for Lewis.
From the onboards, the replays and watching it over, Vettel could have turned left and backed off to give Lewis the position. If you were driving any online race in this manner it would be a clear cut and penalty, even if you had to rally-x to keep the place.
I guess Lewis realised what was going to happen and felt quite sanguine about it. Good psychological advantage to tease Vettel with later in the championship as well. I'm sure Hamilton quickly realised Vettel would possibly get a penalty, so free bonus points and he could take it easy for the last couple of miles. If not, no loss. I doubt he would have been quite as gentlemanly to Vettel afterwards if he hadn't inherited first place
What we don't have, of course, are steering, brake and throttle traces, and all of the onboards. If Vettel could be seen accelerating from the moment he regained car control and turned right towards the wall to block Lewis, if the FIA AND driver steward were all in agreement, it's cut and dried for me.