The trouble for Vettel is that he's now in a similar situation as last season where he is finding himself having to push harder and is subsequently making more mistakes, which in turn means he has to push even harder etc. etc. - although he is undoubtedly still a great driver, he is only likely to face increasing pressure and that's probably not going to be a good thing for him.
It's still possible (if less likely) that Vettel can reverse the current trend and re-assert himself as the lead driver in the team, though ironically, Ferrari probably don't want to see such a reversal, not least because it leaves them open to the 'Hamilton/Alonso' dilemma (where Raikkonen was the ultimate winner in that fight). I reckon if Leclerc can outperform Vettel in the next race or two, then it should be a no-brainer to prioritise him for the remainder of the season.