"You know what Jeff? You should let me use it instead. Either you're pranking me, in which case you'll have even more reason to mock me, or you truly believe it will work and, since you asked me what to change, it would be better if I was directly using it. Don't you think?"
After thinking a bit, said boyfriend concluded that it couldn't hurt to let Jenny have her fun. She was his girlfriend after all and he trusted her. Maybe he shouldn't have.
She programed the Chronivac so her boyfriend and her self would fuse into some sort of monster. One that would mostly look like Jenny already does, with said girl in control of most of it's body, but would have Jeff's mouth were her pussy should be.
That said, the boy's mouth wouldn't look like it used to be. Instead, from the outside it would still look like ordinary vaginal lips. But inside, where Jeff's used to have teeth, there was instead 3 rows of retractable hooks to give them the ability stop any penis entering from ever going out if they so wished. It also contained a tongue that had specially evolved so it could coil around a penis and masturbate it. Somehow Jeff would still be able to speak, and with the exact same voice he had as a human. And all that without causing Jenny and him any orgasm, despite Jenny still having the normal pleasure centers in that region.
Although Jenny would be the only one able to control most of the body, (the lower mouth being the only exception) since it was shared with Jeff, he would hear all she hear, see all she see, feel all the caress she received…
While the resulting monster would be able to eat and live like a normal human being, if its vagina would exist as a weird lower mouth able to speak and with its own mind, its because it would also be able devour any man it having sex with by draining all life from them through their sperm, in a process that let no trace of the victims.
Jenny also made sure that her doctor and a special monster wing at the hospital would have helped her cover what she was since her birth. She also programed that, while her and Jeff would remember the old reality, both of them would love the new one so much they would never even think of going back to the old one.
Jenny pushed the "valid" button.
Now in her room back at home, but somehow sill in front of the Chronivac, Jenny pushed her panties down and asked her vagina "What do you think, Jeff. Should we devour that weird guy, Dan, that recently started stalking us?"
"Oh yes please! I can't wait to plant my teeth in that jerk's flesh!"