United captain Vidic was excluded from the first XI for the Champions League clash against Real Madrid at the Bernabeu on Wednesday after playing the entire 90minutes against on Sunday against Everton.
Ferdinand, who sat out for the game against the Toffees, started alongside Jonny Evans against Real Madrid. Ferguson was not sure if Vidic was 100% fit after just three days of playing and was not ready to take any kind of risk.
"I had my doubts about whether I was doing the right thing or not because he was fantastic against Everton, but I stuck by what we've been doing with him," Ferguson said.
"Three days to my mind was a risk, it would have been the same for Ferdinand, if he had played on the Sunday I would have had my doubts whether to play him on Wednesday," he added.
Evans has been great for United at the back and Ferguson chose him over Vidic to partner alongside Ferdinand, 34, at the Bernabeu. Evans has emerged as a first-choice defender after Vidic, 31, suffered a knee injury which kept him out for over a year.
Praising Evans Ferguson said: "Jonny has emerged as a real top defender and his form has been fantastic. The thing about young players, particularly defenders, when you get consistency from them you know you're on the right track with them."
"I think Jonny has shown great consistency in the last year. If you remember what I said about last year, it wasn't until Ferdinand and Evans played regularly together that we got a run going," he said.
Meanwhile, Brazailian midfielder Anderson revealed that he tried to leave United on more than one occasion.
The 24-year old has been in and out of first-team action since his £17million move from Porto in July 2007.
Anderson has played over 160 games for the Red Devils but has scored only nine goals.