Make her want it.

You approach slowly without touching her. You breathe in, a silly reaction since it's not like you need oxygen, then say: "Sarah?"
She starts and spins on the bed, quickly scurrying away from you. Then she stands so that the bed is in between you and her, her arms raised in defense.
"S-Stay away Linus!" She says, her voice trembling. "Don't try to hurt me!"
You sigh, and lean against a wall. "As if I would ever hurt you. All I've done today is to try and do what's best for you. Don't you see?" You raise your arms in a dual gesture of annoyance and display. "You know my body better than anyone. Don't I look so much better, healthier, stronger... sexier?"
Sarah gulps, tears already streaming down her face. "You're not my Linus."
You put your arms down, fists clenched in frustration. "But I am! You think if I truly wanted to hurt you I wouldn't have done it by now? I want to help Sarah, show you the true way."
She frowns but says nothing.
"I'm strong, fast, I don't need to sleep and I can do what ever I want." You say. "But most importantly, I'm immortal."
She looks up at that, her tears pausing for a moment.
"Don't you see? We can be together forever. We can travel the world, do what we like, be who we like. And if we got tired of each other, we could have whoever we want, whenever we want. Or we could take them together. Please Sarah. Can't you see this is good for us? The best thing that's ever happened to us?"
"B-but you're evil. You... kill people, drink their blood." She mumbles. "You fucked other women..." She says with a sob.
"When you are like me, you will want to fuck other men too, and you can! As for killing people, don't you see how much better we are than them? We live forever and they die eventually. But we don't have to kill anyone, just take what we need, live in harmony. Gwen's master has a beautiful plan."
Sarah lowers her arms. "I-I don't want to die Linus, and I don't want to be without you." You smile, hand out stretched. T
"Then join me, become what you were meant to be." She looks at you for a long while.
"Will it hurt?" She says.
"Pure ecstasy." You say, shaking your head. She pulls in a deep breathe and approaches you slowly. You smile at her warmly as you take her in your embrace. The look of fear and uncertainty never leaves her face. You wrap her in your arms, and despite everything, she buries her face in your chest and begins to sob once again. You put a hand on each shoulder and push her back slightly to see her streaked face.
"Ssh now." You say, and lean in, biting down softly. She gasps, going very stiff for a brief moment. You drink from her, and her blood is sweeter than you could have ever hoped for. Slowly, you feel her become limp in your arms, and before long you are finished. You look to see her sleeping peacefully, and lay her on the bed. Her skin goes pale as her hair takes on a golden sheen. You see her breasts push against the buttons of her shirt as they grow. She moans and moves in her sleep, her mouth opening briefly to grant you a view of her fangs. Her fingers grow wicked talons. Her eyes shutter open, revealing her new blue, glowing, irises. She looks over at you with a smile, and lifts herself up gracefully. She stretches, revealing her curvier form.
"I had such a beautiful dream." She saunters forward. "And I feel so fantastic. You were right, this is what we were meant to be." She slips a hand down her pants and begins to pleasure herself while moving towards you. "We are better than them, we're perfect...." She approaches, putting her arms around your waist, grinding her tits and cunt into you. "But you never said how hungry I'd be." She says, her eyes flashing.
You laugh together, kissing deeply for the first time in you new life.