Here's my idea:
Cid and Shera are married. In fact, it's their anniversary. Which Cid, being Cid, forgot until Shera handed him a little gift-wrapped box, and now he's trying to cover for forgetting. So he tells her that since it's their anniversary and he was such a bastard to her for so long and he's so lucky to have her (wow, hitting every "oh such a sweetie" button, isn't he?), he's going to devote this one night to letting her live out a fantasy.
You may commence with the evil grin, now.
Vincent is in Rocket Town just for the hell of it. He was in the area, would like an actual BED and a shower and a meal that he didn't cook over a fire. Yuffie is in Rocket Town because she's on her way through to somewhere else. "Where" isn't really an issue, as she's really there for the Washing Machine of Doom, which I am shamelessly ganking from Cheloya, if she doesn't mind.
If you aren't grinning evilly already, you should start now.
Shera, noticing some tension between Vincent and Yuffie, tells Cid that she's always fantasized about seducing Vincent and Yuffie simultaneously. And then she claims that she just doesn't think she could do it alone ("There's really nothin' to it, but I simply cannot do it.... ALONE! *music note*"). So she enlists his "help".
Dinner, they spend terrifying Yuffie. Shera makes some very Freudian statements about Yuffie's love of her washing machine. Yuffie tries to ignore it. She also tries to ignore the fact that both Cid and Shera are flirting with her. Shamelessly. Right up until Shera's hands start sliding around in Yuffie's lap. At which point Yuffie freaks out and spills tea all over herself.
Shera responds with some very predatory maternal bits. "Oh, sweetie, let's go get you cleaned up..." "Did you burn yourself?" Yuffie balks at the prospect of being alone in any room, ESPECIALLY the bathroom or kitchen, with Shera. But she can't very well say "OH MY GOD YOU'RE TRYING TO MOLEST ME!" So she just sort of tries to say, "I'll take care of it myself, I'm fine," but Shera has none of it. Still seeming maternal to Vincent.
So Yuffie lets Shera get her alone in the kitchen.And Shera doesn't molest her. No, she does worse. She gets Yuffie thinking about all the "possibilities". And then changes tactics, asking Yuffie about the tension between she and Vincent. Confuses her.
And when Shera starts talking about the six zillion things she's pretty sure every woman in the world wants to do to Vincent, flooding Yuffie's visual imagination with now not just images of Shera, but also Vincent. Yuffie is maybe nineteen and has no built in defense for sensory overload. Which means that Yuffie is now SHOT. She's in. Shera has corrupted her to the Dark Side.
Meanwhile, Cid and Vincent are discussing how crazy females are. Cid keeps finding ways to insinuate that Yuffie is interested in Vincent. Vincent refuses to contemplate this, and changes the topic to "Hey, have you noticed your wife is flirting shamelessly with a 19 year old girl?" Cid replies that 19 years old does not qualify as "girl" and oh, hey, let's arm wrestle so I can break your only remaining arm because my wife's been flirting with you!
Vincent sort of goes "WHAT OHMYGOD PHYSICAL CONTACT OHGOD", not even bothering to think about the validity of this statement. And while Vincent is having his mini-breakdown... Cid snags his arm.
Arm wrestling ensues, but not quite the way Vincent expected. Cid's other hand keeps invading the fight, and Cid's fingers keep sort of massaging his hand. Vincent throws the fight to get out. Unfortunately, that leaves him pinned. With Cid's hand on top of his. Cid's hand slides up his arm, to his jaw. Then Cid pulls him in and kisses him, then claims it was for the hell of it.
Just as Vincent is trying to engage his legs and run away, Shera walks in with a huge bowl of popcorn and declares "MOVIE NIGHT MOVIE NIGHT YOU HAVE TO STAY"
Vincent can't back out without looking like a jerkface in front of Yuffie and Shera, which he'd really rather not do. So he stays.
They all pile onto the couch. Yuffie sits between Shera and Cid, with Vincent on Shera's other side (he's not stupid, there's no way he's getting near Cid again). Shera puts the popcorn bowl in Yuffie's lap. Fun times when Shera's and Cid's hands "miss" the popcorn bowl ensue.
Shera starts kissing Yuffie, then hands off the popcorn bowl to Vincent and slides Yuffie into Cid's lap, where Cid has a great deal of fun ensuring that Yuffie doesn't watch the movie.
Shera moves in on Vincent. She starts slow, leaning on his shoulder and stealing popcorn, occasionally "missing" (and always apologizing). She starts to consistently miss the bowl. Then she starts teasing him, breathing in his ear, playing with his hair, etc...
He becomes increasingly agitated, nearly gets up to leave several times, but each time, Yuffie asks where he's going. When he decides he definitely can't stand it anymore and tries to leave, Yuffie asks him why he has to be so antisocial all the time. She has very clearly spent too much time in Cid's lap--- Vincent can hear it in her voice.
The sound you hear is Vincent's brain breaking.
He asks if Shera and Cid are doing this on purpose. Cid tells him that they are. Shera says that if he didn't bother showing up at their wedding, where he WOULD have been the Best Man, the least he can do is lighten up a little for their anniversary.
Vincent tries to get all prideful and refuse to be an anniversary present. He stops arguing when Cid mentions that being a present involves sex with Yuffie, who is making little noises from what Cid is doing to her through her underwear.
And I think we can all guess where everything is going from there. Suffice to say that Shera gets her present.