I think you´re ignoring or understimating an important factor wich is common to any top sport when it comes to a rookie joining that top league. I´d call it "realizing you´re not the best"
If you like football (soccer) it´s similar to what James Rodrigez suffered when signed in with Real Madrid. He was used to being the top star of any team he played before, with the whole team working for him because he´s always been the best of the squad. But when he joined Real Madrid then suddenly he´s not the best, he´s only one more member of the squad with several players on a much more important role and then he´s the one who must work for the team, intead of the team working for him. That´s a dramatic change for someone whose ego was as high as it could be before, and I´m sure that takes some time apart from a strong personality.
For Vandoorne it´s the same, last 5 seasons he´s won several races each season on FR 2.0, FR 3.5, GP2 and SuperFormula, winning the WDC in FR2.0 and GP2. He´s been a winner last 5 season, and now he has to cope with a car wich can only finish top10 from time to time, and specially with a team mate who is easily faster and more consistent, so much more from time to time the GP2 champion look like an average driver
He need to realize he´s not the best, assume he has to do a lot of work for the team, and accomodate to this new role, and all that while getting used to F1. I think the beginning was specially difficult for him, but he´s getting used and performing much better in past GPs. It´s still his rookie season after all and he´s compared with probably the toughest team mate of the whole F1 grid, even for veterans and WDC like Kimi, Massa, Button...
IMHO Vandoorne position right now is the most difficult of any F1 driver by far