First off, here are the credits for Ladies Night:
Programming & Guitar: CMJK
B.G.Vocal: Gloria Ryann, Chantale Cerisier, Vickie Natale
Vocoder Voice: jusme
Mixed by Koji Morimoto
The rap is not "jusme" on vocoder, like some people think. That's simply not what a vocoder sounds like. A vocoder is FAR more robotic. Forget "it doesn't sound like ayu", a vocoder is almost completely unrecognizable as even being HUMAN. A vocoder is what's heard at the beginning of "Intergalactic" by the Beastie Boys (the robot voice saying "Intergalactic Planetary, Planetary, Intergalactic" if you're familiar), and in "Mr. Roboto" by Styx.
Here's a sound clip showing what exactly "jusme" did for this song. In the first part of the clip, the vocoder voice is the voice-ish sounding synthy/robot-like sound that accompany's ayu's rap, not the rap itself. In the second part, it takes a more background position during ayu's "Ah-ha" parts.
http://www.sendspace.com/file/ib88s0
THAT is vocoder. Ayu's voice is distorted during the rap, yes, but that's distortion (maybe it's a phone filter... kinda sounds like one), not a vocoder.
There are various reasons why I'm certain it's ayu.
It's definitely not just a sample of Jade's original rap, you can plainly hear when they're played back to back that they're totally different performances by two totally different people.
Also, at low qualities and with distortion, ayu doesn't sound like ayu... I'm one of many that was VERY sure that it wasn't ayu singing "Moments" in the Visee CM back when all we had was the streaming version with low quality. So ayu with distortion would sound not much like ayu. Compare this rap to the one in Fly high, however, it sounds like the same person but a few years older.
Plus, the only BG Vocalists were westerners with (as we could hear in songs like Bold & Delicious and Pride) much deeper voices than ayu has, so that rules any of them out.
So if it's not them, and it's not jusme, and it's not jade, by process of elimination it has to be ayu. I agree the voice doesn't QUITE sound like her, but like zoomzoom said, to say that there's NO WAY it's ayu is very very incorrect, and it sounds more like her than the only other 4 possibilities (being Jade, Gloria, Chantale, and Vickie).