But the British veteran, who used to share a manager with Vandoorne, will also be doing all he can to be comfortably faster than his team-mate.
Vandoorne is seen not only as a star of the future, dominant winner of the GP2 support series in 2015, but also the man likely to replace the 2009 world champion at the British team when the time comes - possibly in 2017.
Bahrain will be the first time the two can be measured against each other in the same machinery and in the heat of battle.
Button, the most experienced driver on the grid with 285 race starts and 16 seasons behind him, has been a match for Alonso at McLaren and will be expected to outperform the rookie, who turned 24 last week.
Anything less than that, in normal circumstances, would not be good news for a highly-paid driver who is out of contract at the end of the season.
Button knows that Vandoorne, potentially a far cheaper option, will be doing all he can to impress his paymasters.
In a somewhat strained exchange in the team’s hospitality suite, he told reporters: “Stoffel is a very experienced driver, he's raced here a couple of times and he won here last year in GP2.
“He's worked with this team a lot, tested quite a bit, so he understands the car, he understands the team and the way of working.
“There are a few things for him to learn but more in terms of controls and what have you for qualifying and the race. He knows what a racing car is, I'm sure he'll be fine.”
Vandoorne was flying to Bahrain from Japan, where he is competing in the Super Formula series to keep his racing skills fresh, after Alonso failed a medical on Thursday.
“About to board for Bahrain, a bit earlier than expected but so much looking forward to it! Will do my very best for the team,” he said on Twitter.
Alonso had earlier revealed that he had broken ribs and suffered a lung injury when he crashed in Formula One's Australian season-opener on 20 March.
Original source: Vandoorne debut puts added focus on Button