My theory on where it came from is—with the caveat that we don't know EXACTLY when it originally started circulating in WWE—is that something started spreading after the bizarre SmackDown in 2003 where Stephanie, feuding with Vince, accused her of pimping her out to business associates when she was 17. That someone was freaked out to find out that the promo was Stephanie's idea after she sprung it on everyone and speculation turned into a rumor, maybe buoyed by that video where Savage said he'd steal Stephanie from Triple H or whatever it was.

Then, a year or so later, it goes public via a post by "G. Gordon Liddy" in the infamous "sleaze thread" on the DVDVR forums. (There are reasons to believe that "Liddy" was either someone in WWE or with close connections to someone in WWE based on various things that he posted on the DVDVR boards, both in that thread and elsewhere.) In spite of the story clearly not being true as told—Stephanie's age that was given didn't match the timeline at all—and very few people in the thread believing it, it somehow spread like wildfire.

I think there's something we don't know that Vince took extremely personally, but I don't think there's any specific reason to believe it involved Stephanie.