My feeling is Vettel is very consistent when the car is in the sweet spot for his driving style. It's when the car isn't under him that he appears to start to have problems and when things go wrong, he's more prone to errors. He's proven he can come back from mishaps in the race, but occasionally he suffers from the red mist and will blame others for his own mistakes. This shows that also has a anger management issue that appears from time to time when in the car and the pressure is really on. There have been several instances where he was lucky to receive very light punishments for this behaviour.
The earlier comments regarding Vettel playing politics within the team are accurate. and Ferrari, traditionally, are not good at handling these combinations.
Vettel is completely capable of winning another championship, if the car and team are to his liking. Sadly, Ferrari's leadership since the Schumacher era are not as stable as they once were. Their influence over the FIA also appears to have diminished considerably in the last decade. However, they are far from being outside of the top 3 and are fully able to win.
Please bear in mind that this is just a personal opinion and doesn't detract from the fact that vettel is a proven champion with RBR.