Alonso's contract runs out at the end of the year, but it was believed Belgian rookie Vandoorne had a long-term agreement with McLaren-Honda.
"I have a contract to drive in 2018," Button, the reserve driver in 2017 and 2018, said at Monaco grand prix.
"I think I'm the only McLaren driver with a contract for the coming season," he added.
Realising he may have misspoken, Button then said: "I don't know, it's just a feeling, not a fact. We'll see."
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