Mariah Carey -- I'm Rehearsing, I'm Ready, I'm Fabulous

Mariah Carey is taking her Vegas residency as seriously as anything in her career ... so say the people getting her ready for next week's kickoff.

Our Mariah sources say she will not have a repeat of Japan ... her voice is "on fire." We got this pic of her rehearsing over the weekend with her musical director, producer and other show-related people.

There's lots of voice protection stuff in the picture ... throat coat, tea, etc.

As for preparation, one of her people says, "She's been singing these 18 #1 hits for a very long time."

Her show starts next Wednesday.