I have a question about changing the pitch of vocals

If you have heard any DIPSET songs, you hear that low pitched chick voice in the back in almost every song, how can I do that to a vocal acapella, what im trying to do is remix that song by 50cent. Best Friend, im trying to make it unique so ima take olivias vocals, chop them down and change the pitch and use it for background vocals.

The problem is when i set the pitch lower the voice gets faster, are you supposed to use a different effect or do you set the pitch then change the tempo of the acapella also? if someone can send me a PM reply to this question itll be nice, i suck at production so yeah im learnin still