On his return from a calf injury which has kept him out for two months, Vidic saw red during the second-half for a challenge on Gabriel Giurgiu.
"I've looked at it and his foot is certainly high," said Ferguson.
"I can understand the referee's interpretation of the incident.
"Maybe it's a bit different in Germany, but I thought it was a little harsh. It was certainly a booking."
The decision annoyed midfielder Michael Carrick, who claimed: "It was a dubious sending-off at best really and we're disappointed with that."