I think he had one season left at this level and it would indeed be a side worthy and capable of winning the Premier League. However, I think Phil Jones has looked great with a run at CB.. Possibly our best player this season up until Di Maria came along.
Really disappointing that he got injured, but if he can get over these problems in the near future then I think he can be great. He'll probably not reach the level Vidic was at a few years ago, but he'll come reasonably close and be good enough to challenge for all major trophies. I think we're fine with the CB's we have now for this season, but breaking the bank for Hummels would definitely be a good decision if we can tempt Dortmund.
Blackett is someone I like as well. Can't say he's done much wrong during his time in the side, but we'll have to see what LvG does and we know he likes his left footers to play as the LCB. That could mean first choice for the RCB are Jones & Smalling, with Rojo & Blackett for the LCB. I think Evans is most likely to get pushed out of the side now, as his form has been pretty average for a while in my opinion. Some fans will disagree and not rate Smalling, but I'd argue that even if you don't think he has progressed quick enough, he's definitely at least stayed the same and put in more good than bad. Evans, in the last 18 months or so, has definitely gone backwards.
The main thing that would worry me no matter if we had Vidic, Hummels, Vidal or another World Class talent is that I still think Chelsea will be extremely difficult to pry the title away from this season.