The Basque outfit locks horns with the Primera Division leader on Saturday, and Vela realizes the club could be in for a tough test.
"Nobody really knows what to do in order to stop Barcelona. If anybody knew, they would already have been beaten," the former Arsenal attacker said at a press conference. "We will just try to play our own game, and hope we will see enough of the ball. We might be able to get something from this game if they have a bad day.
"If they're on top of their game, it will be impossible for us to record a good result, though."
Sociedad was beaten 5-1 at Camp Nou earlier this season.