I think it's all about the hunger for success. Vettel want's it more, thus his effort is higher on/off the track, and also focus is increased. That means on track he is more adaptable to changes, better prepared and makes less mistakes. Put that together and you get a driver which uses opportunities and tackles problems better.
If that hunger, for whatever reason doesn't exist, you get the Vettel of 2014 who got beaten by Ricciardo, or a 2015-edition Rosberg, or a 2011-2013 edition Massa (who has his hunger and self confidence restored since the second half of 2014 at Williams).