Just watched a few vids of the incident, both on-board and front/rear views... I still think Vettel could have easily got passed him without incident.
As you exit that right hander, you immediately aim towards the right of the track to set up for the left, which is what Karthikeyan was doing. If he had have continued straight he would've been off the track.
I agree back markers should do everything to get out of the way when being lapped, especially when they're not fighting for position with any other back markers, but in this case it's not that simple given the series of corners it happened in. Karthikeyan had just been lapped by someone else mid-turn, and would have been lucky to see Vettel coming up behind him immediately after.
The penalty doesn't really matter too much in the scheme of things, but in reality Vettel had a shit load of room to the right that he could have been in, instead of leaving nothing more than a car width on a track where a wheel outside the line could end the other guys race..