Can't sing = Mollie and Frankie
Can't sing but confident enough to go with her limited abilities = Rochelle
Can sing = Una
Can sing but having problems = Vanessa
I mean it's actually laughable when you watch them give a live performance, because it's never truly live. I'm not talking about when they do playback or lip sync, I'm talking when you have Vanessa, Una and Rochelle singing live and you have Frankie lip syncing and Mollie singing over a loud backing track; and inevitably you'll have Vanessa fail to hit the big notes because her voice has deteriorated over time.
This is a good example, at 2:45-2:56
I mean it's just sad, Vanessa used to be an incredible singer and now she really struggles to hit notes. Just watch any performance they give nowadays. You can tell when she's struggling because she looks up at the roof and her voice strains so hard; and she's surrounded by people who are only there because of their looks. At least Girls Aloud had three good singers to rely on.