Not 100% sure on what you're looking to achieve... I'm having a hard time understanding what exactly you're after. It the Britney track, it sounds like a few clips of the vocal were chopped up and re-pitched, to whatever the audio engineer(s) set it to... but in Bieber's example, that sounds like a totally different effect - like it's trying to replicate a record scratch on the vocal, where the vocal is going normal speed but slows down immediately at the end of each line. If the latter of the two is the effect you're after, a live time-stretching plugin like GrossBeat would be perfect for this effect; just draw the curved line going down at the very end, so that it only effects the ending lyrics in the line. You could also do it manually by creating an automation clip to control the audio clip's pitch, but you'll need to have set to a mode called "RESAMPLE" so you get that pitch effect when you do it. Hope this helps!