I noticed this last match aswell, and it seems to be working well so far. I think it might have something to do with all of the injuries that Rio have been suffering, and that we've rotated heavily on the right back. With Vidic back, no matter who plays at right back will have the same player covering them at all times. No matter if we play Fabio, Rafael, Jones, Smalling or even Valencia, we'll always have Vidic at their side which means they will more easily adapt whenever they are thrown into the team.
With Evra staying clear of injuries, it's important that the person next to Evra / Vidic will know who they are playing alongst and will get to know their style of play. It's easier to play an somewhat inexperienced player in the centre of our defense when he's got players like Vidic and Evra next to him every time he plays. It's more benefical to play alongside someone as reliable and experienced as those two, instead of having to play alongside a different right back nearly every time you play. Atleast this is how I look at the reason for change, and I think it's very sensible of Fergie if that's the case.