I'd say Schumacher was a megastar. There were some superstars in Hakkinen and Alonso and some others and the rest of the field were stars. I don't think Vettel is yet elevated above the other superstars as a megastar.
It pains me to say it but I see Vettel as a better driver than Schumacher even when Schuey was winning all the titles back to back, reasons:
Schuey had a car far better than anything else on the grid.
Seb is going to this season win his 3rd title, with 2 of those titles when having a worse car than McLaren.
Schuey whenever pushed by a decent driver always punted them off the track.
Seb just drives round the outside of them, e.g. Grande Curve
Schuey always had a schmuck No. #2 who would support him.
Seb has Mr Grit, who's never around to do that, well recently.
(okay this last argument point is a bit lame)
As for 'Megastar', please please no no NO, please do not use descriptive words like that, you're pandering to everything what is wrong with self inflated celebratory status of the media world today.
Schumacher was a great driver in an equally great car, Seb is a good driver about to jump to the Great level (3 titles) in what is a good car (again it itself is about to move to a Great car with it's own title).
Megastar
(if I really have to use it to finish off this argumentative point) is something you associate with a person who is by far better than anyone else at their sport, if you want someone who is anywhere close to being a 'MegaStar' in their sport, you have to look at Mr Loeb in Rallying.