The Belgian, 25, had a difficult start to his rookie campaign as team-mate to double world champion Fernando Alonso, but his form has improved.
With Alonso's future uncertain, McLaren have ensured continuity with at least one of their drivers next season.
"We've great confidence in Stoffel and he's getting better all the time," said racing director Eric Boullier.
"His team-mate is a tough opponent - that's an understatement in fact, because he's arguably the best driver in the sport today.
"But Stoffel's robust talent and fierce ambition make us sure that he'll achieve great successes with us in the future."
McLaren executive director Zak Brown added: "I regard Stoffel as a super-talent - a future Formula 1 world champion in fact - and that's why I've always been adamant that he should race for us on a multi-year basis."
Brown said he would not discuss the duration of Vandoorne's contract but added: "When we signed Stoffel, we intended that he would race for us for a significant number of years, and that remains our firm intention."
Vandoorne said: "I'm delighted that the team has now formally announced that I will continue to race for them next year, because I'll be able to approach the second half of my rookie season with total focus on the job in hand: namely getting the very most I can out of my car, my engineers, and everything and everyone around me."
Alonso, who turned 36 last month, is out of contract at the end of this season and has said he will wait until at least September before deciding on his future.
McLaren want to keep him, and Alonso has said it is his "priority" to stay in F1, where he does not appear to have other options, with top teams Mercedes, Ferrari and Red Bull not interested in him.
But he has also said he wants to win in 2018 and it is far from clear McLaren will be able to satisfy those wishes - whether they stick with struggling Honda as engine supplier or switch to Renault, as is the other possibility.
Alonso has intimated he would be prepared to consider a future outside F1, most likely in IndyCars, where he has an ambition to win the Indy 500, in which he was racing for victory this season in a McLaren-supported programme before his Honda engine failed.
Stoffel Vandoorne will stay with McLaren for a second season in Formula 1 in 2018, the team have announced.