Vinny is out of form and hasn't played 3/4 of a season for 4 years. Mangala hasn't established himself yet and Demichelis is 35 and there's been rumours he'd like to go back to river plate.
Take your pick! It may seem like a slightly luxury signing but it's the kind of signing CL winning teams would make. Real have 3 starting quality CBs, Bayern have at least 4, though their injury record is worse than ours. Chelsea seem to be getting Stones so they'll have 3 starting quality.
If the money is there then it would be a good signing IMO as we know that defenders who suit pellegrini's system are hard to come by and Demichelis has at most a year left in him. We may be moving him on slightly early for long term benefits.