The two McLaren juniors are effectively fighting an exclusive battle for this year's championship heading into the season's penultimate round at Paul Ricard this weekend.
Magnussen outscored Vandoorne by nine points last time out in Hungary to extend his lead to 36 with 100 still up for grabs this year.
And Vandoorne believes that he has to pay close attention to Magnussen if he is to have any chance of beating him at the end of the season.
"I think I should keep focused on Kevin," Vandoorne told AUTOSPORT.
"The gap is not ideal now, but everything is possible if some things go wrong for Kevin and we can win.
"Kevin has the championship in his hands. It's his to lose now."
Vandoorne admitted that his strategy in the second race in Hungary last time out, where the two title foes were fighting with Antonio Felix da Costa for the win, was focused more on beating Magnussen than ensuring victory.
"We wanted to get some points back on Kevin and the race was all about trying to undercut him [during the pitstops]," he added.
"He's the guy we are racing for the championship, but it didn't work out."
Magnussen, who can seal the title this weekend if he outscores Vandoorne by 15 points, believes it is too early to assume that the championship is in his hands.
"We need to try to keep focused on the job," he said. "I'm leading the championship and that's a good feeling, but it's not enough.
"We need to try to win this championship and I'm sure that the others are trying the same.
"So we focus on our job, not getting carried away and keep working hard."
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