It's all our team contributing to the errors though. The non-Vermaelen CBs have both had good seasons on the whole.
Williams isn't a bad player, but he's not a better defender than Koscielny and BFG. And despite Vermaelen being awful recently, he has shown he can be a good player in the past. If we're buying someone like Williams as third/fourth choice, it's not too bad. But he wouldn't improve our defence.
We're not buying him anyway. Just like we're not buying Kalou, Biglia, N'Zonzi, Scott Dann or Trabelsi.