Even if Vettel didn't win another championship, he will go down in history as that kid with rocket boots, who could beat 4 former champions in a very good car made by a soft drinks company.
So in my opinion he is already a legend. I certainly hope that he doesn't stop at two however! Next year he could make it 5 World Champions!
Now - I wasn't alive for much of Senna's career - and I wasn't following F1 for much of Schumacher's - but I'm pretty certain that Vettel is the best ever qualifier to grace the sport. Or if he isn't, he will be.
Hey now I can post my opinion about Vettel and a rumoured move to Ferrari in 2015. We all know Alonso's contract with Ferrari goes to 2017. Would Alonso and Vettel be paired together for a couple of years? Or would they wait until Alonso retires and then sign Vettel.
A thought came to me last week for those (including myself) who have criticised Ferrari's preference for one number-one driver and a driver well behind. Yeah that happened with Schumacher. But it's well known that Schumacher was winning at a time when there wasn't a lot of talent on the grid (there was some). So who would you have picked as Schumacher's team mate if it weren't Irvine, Barichello and Massa?

When they signed Alonso I am convinced that Ferrari's plan was to have Raikkonen as his team mate. Alonso and Raikkonen

But it didn't go according to plan because of Massa's accident. It wouldn't have been fair to oust him while he was out injured.
Theoretically the last time Ferrari had two number one drivers was the Massa/Raikkonen pairing. Before that you have to look back to maybe Prost and Mansell in 90 I think?
But particularly in the 70's and 80's Ferrari did try to have the best two drivers on the field. Pironi and Villeneuve - that partnership and the stories that unfolded between them are etched into Formula One folklore to this very day. Also drivers like Lauda and Sheckter come to mind...
So the concept of Vettel and Alonso at Ferrari seems a probability and likely to happen. For Ferrari's replacement of Massa - they will want someone who has the potential to win races. And Hamilton isn't out of the equation....