Mariah Carey 's clearly not onstage a ton anymore ... 'cause her choreography was a bit awkward onstage at the MTV Video Music Awards -- including a fake slap she really didn't sell.

The singer-songwriter took the stage at UBS Arena in New York Sunday night to perform some of her hits and accept the Michael Jackson Video Vanguard Award -- a lifetime achievement presented at the ceremony each year. Getty

After finishing up a song, a woman in an all-red outfit -- who Mariah called Bianca , a reference to a character from the 1999 "The Heartbreaker" music video -- came out ... and, Mariah pretended to smack her in the face -- all rehearsed choreogrphy.

Well, perhaps not rehearesed enough ... 'cause Mariah's fake slap was off-time -- and, looked pretty cringy. She seemed to heist

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