Playing Shaw as a wing back is absolutely playing him out of position. His attacking is promising but it's his superbly mature defensive game which makes him special at the moment. Meanwhile the idea that Rafa has defensive weaknesses which require us to use him as a wingback is very out of date - he's been a perfectly good defensive player since 2012, and it's only his absences through injury making people forget that.
But the main point is still that if Herrera and Vidal are too attacking a partnership, adding another CB doesn't solve that problem. You need to put a holding midfielder behind them. A CB cannot play as a DM, it's a completely different skill set. A lightweight midfield will still let the opposition run right through them and attack the back line at pace, and having an extra CB won't make much difference in those situations. If you want to stop that happening, you throw Carrick in there with them.
As I say, though, I think they've got more than enough about them defensively. They'd be an absolutely classic box-to-box two-man midfield. Both offensively competent, both defensively competent, both with superb engines (Vidal's, in particular, is probably the best in the business) and both tactically intelligent, mature players. They'd keep the ball well (especially with Mata ahead of them) and when they did lose it they'd be all over the opposition like a bad rash until they won it back.