If being out of circuit bounds gets you a punishment, they'd be handed out to half the grid.
Jenson cut the corner, he shortened the length, Vettel went on the outside - loads of drivers have done that.
You're not being unreasonable, but there seems to be some confusion about the rule. "Cutting the corner" is not a specific offense in F1 at this time. The offense is "Exceeding the Track Limits". In a race, drivers will be warned via their teams for doing so if it doesn't have a material impact. If, however, they "
maintain or gain an advantage" by exceeding the track limits, they can be issued one of the Stewards' standard penalties or asked to yield to the driver they were attacking/defending against.
"Exceeding the track limits" is defined as having all four wheels outside the white (In this case, yellow) lines defining the track edge. Vettel carried so much speed into the Turn, he ran completely wide and all four wheels left the track. That's the exact same offense as Jenson was booked for, just with a different cause. Similarly, Buemi is guilty of almost exactly the same thing exactly where Vettel did it.
The fact is, vettel's overtake would not have stuck had he braked appropriately to keep his car ontrack. Indeed, had the runoff been grass or gravel or worse, had their been a wall, he'd have lost tonnes of time or been out had he pulled that stunt.

The Frome Flyer: Smoother, Smarter, Calmer,
Winner.
Jenson Button: Professor, Chauffeur, World Champion Racing Driver.