I really can't see the point in moving Fellaini back up there.
Not that I don't think he'd do a decent job again, but that he's now too important to the team in the role he currently has, especially given the 2 CBs we have to play at present. Neither are particularly dominant in the air so it's important that he's there to help them out.
The only reason for moving him up is that the Yak needs someone who heads the ball and puts in a shift up next to him, but since he's off soon there's no desperate need for that particular situation.
I'd much prefer to move Ossie up there, at least until Vaughan is able to start PL games.
Vaughan could be key to the rest of the season as he provides all the aggression, work rate etc that Cahill does but can also add pace. If (IF!) he stays fit, I think he's the only forward we have that allows us to play 4-4-2 to the same level as we play 4-5-1 as he allows the other forwards the time and space to do what they do best.