Threshold

[makeshift]

Threshold 

Once, when she gave permission I always found this part of the vampire myth fascinating, and i'm glad to see your poem open with it. Once or when could probably be omitted. Once brings me to a fairy tale, i.e once upon a time. 
It was a vampire who dipped his toe
across the threshold. Threshold is nice because it has so many different meanings. Threshold here is especially weighty because of the repetition with the title. I do think Threshold is a good title, but it works better with this first stanza then it does with the poem as a whole. 
She smiles, unsuspecting. He stays-
because she can't make her mouth form the words
to make him go. I get the idea here that her mind and body are mis aligned. You could consider "Because she can't make her mouth form the words/"go" but maybe thats to melodramatic or cute or something. 
She dances like a snake for him
In one light on the dance floor
Deadly and beautiful, her eyes are shadowed, top lit
He doesn't need to see them to know her thoughts I like the scene described in these four lines, but cant help to think it could probably be reduced some. Deadly and beautiful are notions already expressed by the image of the dancing snake. You have dance here twice. Maybe see if you cant get the shadowed eyes and one light in one stroke?
Windows to the soul and all that--true, yes Cant decide if the selfawareness of the cliche excuses it for me
But even sightless--there is touch
something electric in the fingertips
he drags along her collarbone. I can feel this, very visceral 
Telling her things
Turning her on, telling her things 
Watson, come here I need you Is this referring to Watson X Sherlock Holmes? Watson is really unsexy 
Breath like a stone,
Skipping Last two lines are my favorite of this first stanza, a stone is such a surprising vehicle to express breathe, but it works well.

Is it her fault the vampire
finds her beautiful? Her fault she's 
Hooked by the way he hurts her?
Betrayal--in the eyes
A pebble down an empty well.
There is no love in those eyes
green, roiling with a venom she can't expel
It turns counter-clockwise in the stomach
Wanting out, words pushing against the back of her teeth
But don't...but don't. Shh
His gentile fingers at her temples
blinders calm the filly-
With his touch the junkie gets her fix You ever read William Burroughs' Junkie? It has a lot of ghoulish imagery, more abject then what you've written, but could be a source of inspo. 
Somnolent pliable bliss
She holds his face like a loving cup
Precious,delicate
Forgiven, forgiven
Yes

Carousel The carousel is an effective metaphor here, whith a nice rythm to it. 
So good, so up and down
She loves the ride, loves the thrill
Loves the thought
He curls his toes into her gray matter, I have a hard time envisioning this one. 
scans her thoughts from above
Highlights in neon, favorite secrets
Chuckling. While she is laid bare, just laid
Bleached like a bone in Arizona These four lines I do visualize well. The neon highlights vs the bleached bone, its real pretty, but also v unnatural in a way that makes me feel uneasy. 
well past death
But the memories of it were so good,
So up and down, she goes often
Pays willingly, digs deeply
Fingers clawing in her pockets
For the fare. Its a gr8 ending I think, you were really able to make the fare paying business grimey. It works well as a metaphor as well.

What I enjoyed most about this poem was how it was able to pull me into its world. I really could get a sense for the manipulation or trance this girl is under. I felt like your second stanza was the weakest, it offers less to hang on to. You seem to have a natural sense of rhythm in your writing, but I wonder if the repetition is doing much for you, mostly looking at the second stanza here: 
Forgiven, forgiven   
But dont... but dont..

Thnx for sharing!