On the one hand, other than a few mistakes he's been very solid. If anything, his mistakes haven't costed us games. In the big picture, our inability to attack or defend well as a team is to blame, and the manager is largely responsible for that.
On the other hand, he's had a mistake almost every game he has played. and while we play badly, he has often been the player responsible for conceding a goal that has lost us points. on top of that he is our highest earner - so there is an economic advantage to terminating the contract and recruiting (eg.) rolando on a low wage.
sorry, but the second argument is stronger...
edit; there is the question of whether or not this is an adaptation issue (to Italy - new culture, Inter - new team, and the 3 man defense), but can we risk vidic making more mistakes in the present if he even comes good in the future?