Ferdinand, 32, has found his starting place alongside Vidic increasingly under threat due to injuries and the emergence of youngsters Jonny Evans and Chris Smalling.
"I explained to Rio other day that it's simply we have two young centre-halves coming through and I'm very glad to have the four of them and I hope I can keep the four of them for a long time," the manager told a news conference on Tuesday.
"Rio's almost 33 and he's obviously lost a yard of pace he had five years ago but that doesn't mean he can't tailor his game in a different way - where used to rely on pace he doesn't do that now, he can't do that any more.
"We've all faced that decisive moment in your careers when you realise you have to change your game. I did it myself as a player... Rio will do the same and he will be fine."
Ferdinand, who played in the 6-1 thumping by Premier League leaders Manchester City at Old Trafford last month, will return to the scene of that drubbing for Wednesday's Group C encounter against the Romanian champions.
Ferguson said the England defender still had a "big part" to play at United, where the likes of Ryan Giggs and the now retired Paul Scholes have perfected the art of prolonging their careers at the highest level deep into their thirties.
Giggs, 37, has done it by cutting down on games and moving to a more central role from the wing but is still called upon to start some of the biggest Champions League and Premier League matches.
TOP CLASS
A shift in Ferdinand's role at the club was already evident last season when Vidic was made captain ahead of the Englishman, who had up until then often worn the armband when then skipper Gary Neville was sidelined with injuries.
"In his time with us - eight years now - he's been a fantastically consistent performer, top class, absolutely brilliant," Ferguson said.
"He will rearrange his game a little bit, not drastically. He is still one of the best footballers in the country in terms of use of the ball, he's still got great presence, he still can tackle, he still can head the ball. All that's changed for Rio is that extra yard he had two or three years ago.
"He'll change, there's no problem with that, and he'll play a big part."
Ferguson added that Vidic's red card against Galati in Romania a fortnight ago had been harsh but understandable. The Serb now starts a two-match European ban.
Midfielder Tom Cleverley has been ruled out with an ankle injury, joining Smalling and winger Ashley Young on the sidelines.
The English champions are second in the group on five points, two behind Benfica and one ahead of third-placed FC Basel. Galati have yet to get off the mark.
"We'll make one or two changes, not a great deal of changes, that's because of injuries and things like that," Ferguson said.
"I think that from the experience of the last game they [Galati] were very organised, very committed, a very strong, aggressive team. Obviously we need to put a good performance in to beat them."
