You turn Martin, then set him on Juliet.

You'd love to see the look of fear and betrayal in Juliet's pretty face. Tragic beauty indeed. You march up the stairs. The couple sees you coming, but its too late. You look Juliet straight in the eye and shout: "Be still!" And she obliges, powerless before your command. You grab Martin, who valiantly beats at your chest, and sink your fangs into his neck, drinking his warm blood in heady gulps. Satisfied you drop him to the floor, then turn to Juliet.
"Okay you can move now." You say, and she starts, fear and shock in her eyes.
"What did you do to Martin?!" She begs. Behind you you hear movement and grin.
"Don't you think the question you should be asking is; what is Martin going to do to you?" She looks at you, the fear and realisation growing in her pretty eyes, then looks at the vampire over your shoulder. You lean back to allow Martin forward. He grins his razor sharp smile at his girlfriend.
"Oh Martin..." Juliet says as tears roll down her face. "What has he done to you?"
He laughs; "He gave me a gift Juliet. One I'm going to give to you!" He pounces forward and grabs her by the hips. She beats at his back but he only laughs. He pulls down her tight jeans, revealing her large round ass and skimpy thong.
"Mmm yes I'm glad you dressed sexy today." He says, pushing the thin fabric aside with his cock. She yelps and gasps as he begins to thrust.
"M-M-Martin! Your too big now! P-p-ple-!" Her sentence is cut of with a loud groan as she separates her legs, trying in vain to widen her mortal pussy.
"Don't worry." Martin says. "When you're a vampire you'll be able to take my cock easily." Then he bunches up her tunic, revealing her large, heaving breasts, held in place with a flowery, lace bra. He snaps it away with ease and slathers her large areolas with his long tongue. Juliet is still crying, but the moans of ecstasy are undeniable. Their pounding continues, Juliet's moans rising in volume. Finally, she yells.
"Fuck me! Fuck me with your fangs mi amore! Por favor!" Martin obliges, digging his fangs into her breasts as he comes. She screams, the orgasm jiggling her tits which become firmer as the change begins. Her buttocks becomes rounder and larger, giving her a ridiculously curvy figure. Her slightly flabby tummy tightens and becomes muscular and well defined. Finally, her canines extend into cruel tips, and her eyes glow a deep crimson. She breathes heavily, as does Martin who nuzzles her breasts. You approach, and pat Martin on the back in congratulations. You give Juliet a hand up and pull her into a kiss.
"Mmm thank you for turning us Linus. I think we were about to get away too, weren't we Martin?" She says, extending a hand to her boyfriend. He takes it, embracing her closely.
"Yes we were I think. Good job we didn't though! Thank you Linus." You smile.
"My pleasure." You say.