Teen Mom Mackenzie McKee -- I Have A Sex Tape But You Can't

Mackenzie McKee has gone to Vivid Entertainment about a sex tape, telling them if they dare release it she will nail their ass harder than Farrah Abraham's ever was.

TMZ obtained a letter Mackenzie's lawyer fired off to the porn giant, acknowledging there's a tape but warning it will remain private, or else.

It's unclear how Vivid got hold of the tape, but here's what IS clear ... the only way Vivid can release the video is if she signs off, and her lawyer has given Vivid 10 days to declare its intentions.

As for her sex tape partner, it's unclear who it is, but she's been a married woman for 2 years.