Earlier, the British team expressed concern about how the Belgian rookie was struggling to adapt to life in formula one.
"Unfortunately for Stoffel, he was at the centre of the attention of the press and spectators as he became the teammate at McLaren of a world champion," team boss Eric Boullier now tells the French broadcaster Canal Plus.
"But everyone who made their debuts in recent years, including Max Verstappen, went through several difficult months before they began to reveal their potential, but it was less noticed," he added.
"During this time, Stoffel learned a lot and he's now doing an excellent job. He is completely confident in himself and is always gaining more experience," said Boullier.
Vandoorne, 25, will continue to race for McLaren in 2018.
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