Vidic was sent off for a high challenge in his comeback game.
"I saw the incident from behind. I didn't expect the red card," Smalling told MUTV.
"Referees are sometimes more harsh in Europe but I'd have to take another look at it [on TV, to judge it properly]. I think for the first five minutes after we went down to ten men it was a bit frantic but then I think we started to settle down again and control the ball again. Luckily we saw the game out."