Allegri said the choice was for technical reasons despite Vidal being fined for allegedly getting involved in a street scuffle outside of a nightclub last week.
He told reporters: "Vidal's omission against Roma was a technical choice. He'd played a lot of games and I had [Andrea] Pirlo.
"What happened off the pitch has nothing to do with it, that's a problem for the club. Now, however, he must have a different attitude."
The Chilean has been heavily linked with a move to Manchester United in recent months.