Junicane

Danny and Vlad reluctantly agree to live each other's lives to the best of their abilities for a day, since they both have pressing engagements they can't miss, and then beat up the mind ghost who switched them around at the beginning (they were fighting in the Ghost Zone and disturbed Psyche, who promptly zapped both of them so they could "learn to see past their differences and get along"). 

During that day, Vlad goes to Casper High and is immediately found out by Sam and Tucker, who take it surprisingly well, besides Sam's totally unprovoked burst of violence. Before going to class, they try to bring a wildly out-of-touch Vlad up to speed on what kids are into these days.

Vlad directs a snide remark towards Dash, who barely understands what he said but stuffs him in a trash can anyway.

"Some things never change, huh?" Sam says.

They're doing a Shakespeare unit in Lancer's class, and it just so happens that it's presentation day. Danny insisted that he can't miss it because if he does it will tank his grade, and Sam in particular threatens that "he'd better not ruin this for Danny, or else." Vlad glances over Danny's note cards, rolls his eyes, and launches into an extremely dramatic recital of Hamlet's "To be or not to be" monologue. When he's done the entire class is staring in varying degrees of WTF at him, including Lancer, who wipes tears away and proclaims "Midsummer Night's Dream! If I didn't know any better, I'd think you were possessed by the spirit of a true thespian, Mr. Fenton!"

"Why, thank you, Mr. Prancer," Vlad replies modestly. 

At lunch afterwards, Sam demands answers. "What, you're a Shakespeare buff in addition to being an evil billionaire supervillain? How many cliches do you need?"

"I am a man of many talents, Samantha, and I don't appreciate being labeled as a supervillain," Vlad replies without a hint of irony, sitting in between her and Tucker and displaying no interest in the tray of cafeteria slop in front of him.

"Oh, I'm sorry! How about 'creepy criminal dirt bag'? That any better?"

"So," Tucker interjects mildly. "How long were you in your high school's drama club?"

"All four years," Vlad says without thinking and then looks ruffled that Tucker clocked him so easily.

"Ohhh," Sam drawls. "Yeah. That makes almost too much sense."

Vlad goes on to recount the story of his high school's Hamlet production during his senior year, how he obviously had auditioned for the title role and poured his heart into it, only to lose the it to a freckly, snot-nosed freshman because the director was his mother. 

"I have never forgiven them," Vlad says, brimming with decades of unresolved theater kid angst. 

"Uh huh," Sam rolls her eyes, completely unimpressed. "So who did you end up being? Tree no. 2?"

"Nah," says Tucker knowingly. "Not Tree no. 2. You got cast as Hamlet's creepy, murderous uncle, didn't you?"

Vlad twists Danny's face into something absolutely medusan and looks like he wants to say something, but doesn't.

"I'm not hearing a no, dude."

Meanwhile, Danny stumbles his way through corporate meetings, gets tired of spending Vlad's money on dumb stuff, realizes just how empty, lonely, and boring his mansion is, and finds himself on FentonWorks's doorstep. He spends a great deal of this adventure hanging out with his own parents as Vlad, which barely ranks in the top 5 weirdest things that he's ever done. 

Back at Casper High, Sam and Tucker have to practically throw Vlad at the attacking ghost of the day, which he complains about the entire time.

I haven't fleshed out the climax of this story as much, but Danny and Vlad manage to trip their way through fighting Psyche and all is made right in the end. The two of them elect to learn absolutely nothing from this incident specifically to spite the mind ghost and continue with business as usual.

EPILOGUE:

"It's good to have you back, man," Tucker says to Danny the following day, somewhere in one of Casper High's hallways.

"It's good to be back," Danny agrees. "Thanks for keeping Vlad in check while I was gone, guys. That must have been an ordeal."

"Uh," Sam says, but is interrupted by a few other students walking by.

"Hey, Danny!" one of them says. "Wow, dude, we're still reeling from from the other day. We had no idea you could sing that well."

"Yeah, congrats on landing the title role!" the other exclaims.

A sinking feeling begins to form. "Um, guys? What are they talking about?" Danny asks, but then Mr Lancer appears and begins shaking his hand enthusiastically.

"Mr. Fenton, we on the faculty had no idea you had such talent, but after yesterday we expect great things from you, young man!"

"Guys, what did Vlad do?!" Danny hisses, and Sam sheepishly rubs her arm while Tucker looks at the ceiling awkwardly. "We tried to stop him, Danny. Honestly, we did."

YESTERDAY. Vlad, in Danny's body, freezes midstep in the hallway, a bulletin catching his eye. He reaches up, intent, and pulls it from the wall. It's a colorful poster with bold lettering and a pixelated jpeg of a distinctive white theater mask. CASPER HIGH PRESENTS: THE PHANTOM OF THE OPERA. AUDITIONS TODAY ONLY!

A smile-- tight-lipped, wide, and unmistakably, gloriously evil--spreads slowly over his face.

Meanwhile, Vlad sits at his desk with his cat in his lap. The phone rings. He snatches it off the receiver furiously.

"Hi, this is Bob with the Loony-Bin Psychiatry Clinic. I'm calling to confirm your Thursday therapy appointment--"

"Cancel it," Vlad snarls and hangs up.

The phone rings again. For the thirtieth time.

"Oh, for the love of--!"