Nicki Minaj -- BS No Time to Zip ... Wardrobe Malfunction STAGED

Nicki Minaj CLAIMS her dress was unzipped during her performance at the VMAs because she didn't have enough time to get ready ... but that's BS -- she pulled the same stunt during rehearsals.

Eyewitnesses who watched Nicki's rehearsal Sunday morning tell TMZ ... she walked on stage wearing the same dress -- unzipped. One eyewitness says it caught her eye because she initially thought Nicki had come out in a robe.

We're told Nicki was wrapping her arms around the dress in a "weird" sort of way ... trying to manipulate it to see how much would be exposed.

And we're told ... the VMAs created a make-shift dressing room right off stage -- draped with black sheets -- so she would have enough time to change.