1880-1910 seem practically interchangeable to me.
Here's the reply I've been working on for the past hour...
Like Lt. General Hansen said, I'm also thinking since you already have the vague contents of the plot, you should focus on the structure so you can see where things need to be expanded or reduced, etc. You need to nail down the timing - the plot points, climaxes, specific obstacles - what is the inciting incident?
But since you're having a hard time separating Cinderella from it, you need to do some major focusing on Hazel instead. Stop thinking about it being a Cinderella story at all, completely ignore Josephine for a while, and just focus on Hazel - who is she, what does she want, what are her strengths and weaknesses, how is she going to grow or change. If you're still struggling, maybe you need to create some distance from the source story - maybe he's not a prince but his father is Governor of...Virginia? And Hazel is... a southern belle and Josephine is her cousin from...Barbados...who was recently orphaned in a tragic accident... Make it your own and all about Hazel.
Also...
What if Hazel doesn't know the milkboy is the prince right away but the audience does? Depending on the scenes/acts, hiding it from the audience may not flow very well? How much of a "thing" do they have if she thinks he's a lowly milkboy while she is high society? (Reversing the situation in Ever After where he thinks she's a lady only to discover she is not.) What's the reason for the ball? A ploy on his part to set up the big reveal and confess? Only he gets bitten and obsessed either just before or just after the big confession which puts a huge wrench in their HEA? (That part feels a little like Swan Lake with the prince getting decieved by Odile...

) Marrying her out of gratitude seems anticlimactic if they already had a relationship as milkboy? Plus who wants an HEA out of obligation?
Is the audience supposed to know Josephine is a vampire right away? Seems to me the story would be more thriller-like if things are ominous and she's scary but it isn't revealed she's a vampire until the moment she chomps on the prince? Or even trickier, if somehow the reveal is kept underwraps until the big climactic scene which ends in the staking? A thriller needs to have a central mystery so keeping Jospehine as mysterious as possible until the last moment will only strengthen the suspense.
Here's a good article about Thriller structure:
http://actfourscreenplays.com/screenwri ... -thriller/