Vidal has been struggling with a knee problem, while defender Gary Medel is a big doubt for Chile's World Cup last-16 clash against the host nation at the Estadio Mineirao.
Speaking at the pre-match news conference ahead of that game in Belo Horizonte, Sampaoli said: "Arturo will play against Brazil. He is nowhere near 100 percent fit, but he's a guarantee.
"For Juventus in the Europa League [last season] he played at 30 or 35 percent fit and he's the kind of player who can do that. He's doing everything he can to be involved. Against Spain he also made an enormous effort to be ready for the game."
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The news on Medel was less positive, however, with Sampaoli set to make a last-minute decision on the 26-year-old's fitness.
"He has a muscle problem and we'll have to see how it develops," the Argentine coach said. "If the game were today, he wouldn't play. But luckily both he and Arturo are fast healers, so we have hope. We'll see how he is tomorrow."