Britany Metal

how do you get the vocals by themselves?

I'm thinking maybe they heavily filtered the full mix to make the vox pop out as best they could, then just buried the other original instruments under the rock stuff. Maybe like the opposite of one of those vocal cancelling dealio's. My Fiance has some Britney Spears stuff, and while I can't say I'm entirely familiar with the song of the second video, there are some things sort of in the background that sound like part of the original mix.
 
Probably got acapellas from b-sides / record companies (it's amazing what you get if you ask..)
 
Hey I just learned something!

If you seek Amy.

Now phonetically : F-U-C-K Me!

Love me hate me, but can't you see what I see?
All of the boys and all of the girls are begging to if you seek Amy ( or All the boys and all the girls are begging to F-U-C-K me )

Ah, Britney. <3

Hah, yea, My fiance got her copy of the album from a friend, so without the booklet we figured the song was actually saying "F-U-C-K- Me", never knew it was actually called "If You Seek Amy". Pretty clever, haha.