I think the severity of penalty handed during the race has been somewhat shadowed by how the race unfolded. In particular, Hamilton developing an (unrelated) issue with his headrest, and rejoining the race behind Vettel has given the impression that Vettel was handed a light penalty during the race.
However, if we look at the penalties given in recent years, stop-and-go penalties are fairly rare. Typically, "causing a collision", if penalized during a race, leads to 5 or 10 second penalties. (see e.g. Rosberg in Austria last year). In contrast, Vettels 10 second stop-and-go (effectively a 30 second+ penalty) is quite severe (and rightly so).
Although he could have been given a more severe penalty during the race (in particular the 3 penalty points seem light), I don't think it serves any purpose to further increase that after the fact.