You let Evelyn have her, then join in the fun after.

You're in the mood for a show. You close the door, ignoring Diane's whimperings, unzip your pants, then lean against the door, ready but resting. Evelyn tugs your erect cock once in thanks, then saunters forward, her hips swaying exaggeratedly, no doubt for your viewing pleasure. Diane is properly crying now and has pressed herself on the far wall, trying to get as far away from possible from the sadistically smiling Evelyn. The goth chick grabs the desk, then smashes it into the wall out of the way. Diane screams and pulls her arms into her chest and face. Evelyn laughs and approaches, grabbing the woman by the wrists as she tries in vain to squirm away. Then she slams the woman against the wall, dazing her slightly, and arrests Diane's mouth with her black lips. Diane's protests are muffled but the tears continue to roll down her cheeks, her mascara forming a soggy mess on her face. Evelyn pulls back.
"Now." She says, looking deep into her eyes. "You are going to behave, aren't you?" Diane's eyes fill with panic as the supernatural command bores into her mind, but eventually she nods and stops trying to get away.
"Good." Evelyn says, pleased with her first use of that particular ability. Then she moves her hands to Diane's shirt straps and begins to slide them off her lovely shoulders, ever so slowly. Diane still behaves, but her tears continue to roll. Evelyn then pulls down the top, popping out one breast at a time. She gasps.
"Look Linus! No bra! You naughty, naughty girl!" She slaps one of Diane's small pink nipples, then gives it a tender kiss. She passes the crop top back over them and Diane's head, tossing it on the floor somewhere behind her. She examines her pearlescent tits again, massaging her nipples slightly. Then she buries her face between them with a deep sigh.
"You have no idea how long I've wanted to do that." She muses. Then she pops her head back up, her eyes filled with mischief once again.
"You look so scared Diane." She says, licking a salty tear from the woman's face. She turns to wink at you. "Lets see if we can go two for two."
She leads her hands down the woman's body, starting from her breasts, slowly crouching as she does, jutting her round ass out to you invitingly. Then she tugs violently at Diane's shorts, eliciting a sob from the tormented woman.
"Two for two!" Evelyn cries, and you see that Diane's pink, shaved pussy is unimpeded by underwear.
"Ooh you foxy minx." Evelyn says, as she moves closer to the woman's cunt, arms wrapped around her muscular thighs. She licks her labia gently, teasing her with her long, agile tongue. She pushes deeper, and Diane tries to muffle her moans of pleasure. Evelyn pleasures her harder and faster until she is all but devouring the woman's clitoris. Diane lets out a loud screech, and reaches climax, her breasts and crotch convulsing with each wave of pleasure. But before she has time to finish, Evelyn has tugged her by the ankles so she slides painfully to the floor. With Diane's back on the ground, Evelyn grinds her dripping cunt over the woman's mound, up her navel and tits, finally resting on her gawping mouth.
"Lick!" She yells, her voice thick with vampiric power, and begins to ride the woman's face, hard. She bucks and sways, grinding her cunt in as much as Diane licks. Shortly, she comes hard, her quivering cunt dripping vaginal juices on to Diane's beautiful lips. Evelyn rests on her knees with a sigh, but Diane's crying is still audible. She looks down, smiling sweetly. Sliding her self so that their cunts are touching once more, Evelyn strokes Diane's weeping face.
"It's time, don't you think?" Evelyn says. Diane sobs harder.
"N-No! Please, please, please-!" But its already too late. Evelyn adjusts her self above Diane's chest, and dives down, plunging her hard fangs into the woman's heaving breasts. She screams and writhes, trying to shake Evelyn off, but the vampire is a rock. Evelyn then drags her fangs down deliberately, opening up the wound more, causing blood to spill over her chest. Diane screams again, but this time she is quieter. Her squirming subsides as she loses energy, and her eyes droop closed. Evelyn gulps the last of the blood from her veins, but not before a large amount cakes the woman's tits and pools in the valley of her breasts. You expect her to lap it up, but instead she leans down and smears it on her own tits, moving up so that it covers her navel too, and finally sits her pussy in, making sure to cake her thighs and inner folds. Then she stands and turns, looking at you seductively, sucking one bloody finger. You grin and are, of course, completely hard. You walk forward, and bend over slightly to take her breast in your mouth. The blood is sweet and you find yourself tirelessly lapping it up, your long tongue ranging all over her huge pale tits, teasing at her pierced nipples. You move to her navel, gulping and licking, fondling her belly piercing, seeking out every scrap of blood tucked in the recesses of her hard body. Then you move to the main course. You pull back to observe her crotch region. Her thighs are sparsely covered, but she has really worked the blood into her cunt folds. A single large, crimson droplet drips from her clit ring. You catch it in your mouth, then move to her thighs, looking up to her horny crimson irises. Working methodically, you clean her crotch, seeking each drop hidden among her sparse pubic hair and taint. Then, you tease at the folds, finding that as you move it with your tongue it agitates the liquid, causing yet more to flow out. You thrust your tongue down vagina, forming a plug with your lips and her flesh. The taste of her mixed with that of the blood is intoxicating. You drink, seeking it in every nook and cranny, exploring inside her thoroughly. She moans and sways slightly. Finally, you move to her clitoris, and begin to clean her piercing with your lips and tongue. She bucks and sways against you, the stimulation clearly very severe. The blood is gone now but you continue to pleasure her. Suddenly, you feel something wet envelop your penis. You look down to see Diane between Evelyn's legs. Your cock is between her beautiful, plump lips, and you can feel her long tongue pleasuring it even as she moves her mouth up and down your length. She looks up at you, her neon blue eyes almost making you come right then, and winks. Her once golden hair is now much more white and spreads over her pale back in artistic curves. While one hand grips your buttocks, the other is under her, no doubt massaging her cunt vigorously. You return your attention back to Evelyn as you push your cock into Diane's mouth, the three of you assuming the same rhythm. As Evelyn comes, her voice screeching as her pussy quivers in your mouth, you feel your time coming. You pull Evelyn down behind Diane, who gets the hint and pulls up, mouth still on your penis. Then, just in time, you push Diane back against Evelyn, revealing her beautiful new body for the first time, and spray your massive load on the two girls. They laugh musically, Diane getting it all over her cunt (which she is still mashing furiously) and Evelyn jerking forward to catch in her mouth and on her tits. You and Evelyn then close on Diane, her fondling her hair, you stroking her ankles gently, as she brings herself to completion. You thought she was perfect before, but somehow she looks even better now. Her tits are even bigger and jiggle furiously with each thrust of her taloned fingers. Her hips come out the same as before, but her buttocks are now rounder and more firm, further accentuating her hourglass shape. The muscles on her stomach form beautiful lines which ripple as she nears her climax. Her face is even more refined, her sharp cheek bones complimenting her sculpted nose.
"Yes!" She screams as she comes. Evelyn wraps her arms around her, and you join the embrace from the front. The new vampire bucks and screams between you, experiencing orgasm after orgasm. Finally, you lay together, utterly satisfied and happy.
"Thank you." Diane says. "Thank you so much." She looks up at you, tears of joy in her eye.
You smile down at her magnanimously. "It was truly our pleasure."