I'm a Britney fan since 1999, and I STRONGLY believe Britney sang every song BEFORE all these Britney Jean tracks (I'm not saying she can't or she doesn't , but it's obvious that in the final mixes/album versions her vocals aren't there!.
Just take a listen at BTI, even when the song is pretty much a Keri Hilson invaded track you could still tell Britney was singing throught it along with Kerivand the recording session stems are the proof of this
But with BJ you can't tell where Britney's Vocal tracks stop and where someone else's take over the song at all because the dumb mixers/producer or whoever was in charge of the album didn't care to replace her lead vocals instead of multi-layering all the vocals as they did with Womanizer.
When the stems for some of those tracks leak you will come back and quote me. Btw, Perfume and Don't Cry were one of the few tracks safe from all that mess.