They have access to telemetry right? If Hamilton braked significantly harder than normal at that point, he should have got a penalty for endangering another driver. If the stewards didn't have the data at the time, a retrospective penalty seems in order.
I think the stewards job, in this case, is to award penalties which take effect during, and pertaining to the race. If they see fit, they (or the FIA) should also be able to recommend retrospective action.
The thing is, if they find that Vettel deliberately drove into another car, in anger, then I believe that would constitute "Bringing the sport into disrepute" or something similar. You just can't allow stuff like that to creep into sport, of any kind. It sends out the wrong message.
Sport, and therefore motorsport, should be (amongst other things) a tool to teach values to young people - hense the word sporting. Smashing into somebody because you're angry with them, is not only dangerous, it's unsportsman like and should be discouraged. The higher the profile of the sport, the greater the need for fair play.
Fair play should be seen to be done. If they're deemed to have broken the rules, throw the book at em.