Actually I don't get the impression Vettel was stronger in the second half compared to first half. Just looking back - Vettel was outracing Ricciardo in Malaysia, also in Monaco before hit by car issues. In Canada they were neck and neck before Vettel lost out with strategy games.

The difference is that early in the year Vettel had so many car problems his performance was hard to read, while in the end he had less problems. What is clear is that Vettel didn't perform well in Bahrain, China (early year), Hungary, Belgium (mid-year), UAE (late year) - compared to team-mate anyway.