City appealed referee Chris Foy's decision, but the FA came down against the Belgian.
"In 30 years I've never seen a sending off for a tackle like this," Mancini said.
"Last week I also saw three or four players in the Premier League do tackles worse than Vincent and the ref did nothing, no yellow card, nothing.
"If there was danger, Nani probably would have said something, especially to the referee. He didn't say anything."
Mancini criticised United striker Wayne Rooney, who appeared to appeal to Foy to brandish a card.
"He did this. Nani was there, he said nothing Vincent played the ball," Mancini said.
The Italian himself was criticised for appealing for a card during City's win over Liverpool last week.
"Yeah, I did it. Everyone said apologise as in England we don't do this, so I apologised," he added.
"In England, refs make incredible calls. Sometimes they send off for nothing. I don't understand."
Manchester City take on Liverpool in the semi-finals of the Carling Cup tonight.