Think about the reason you're writing your story. Are you writing for the sake of a love interest? Do you want to insert yourself into the world of TVD? Do you love the show so much you want to be apart of its creation/recreation?
All good reasons.
What's not a good reason is replacing an existing character with your OC.
I don't say many things are wrong with it comes to writing fanfiction. Most of the time, the world is your oyster. Or some crap like that. But, DO NOT substitute your OC for another canon character. Don't steal their plotlines. Don't replicate their relationships in every aspect. Don't steal all of their lines.
It's not interesting. And quite frankly, it's copyright infringement. No one wants to read the show (TVD, TO, L) word-for-word, except with another character in place of another. At least, not the readers I know.
Now, I was guilty of this when I first started out. I had to learn how to manipulate the storylines of the show, but still maintain the pivotal points and character arcs that are necessary for everything to continue.
It's harder than it sounds. I'm still figuring it out.
Create your own scenes. Write your OC doing things THEY would do, not another character.
If your character's a humanity-less vampire, have them go on a blood binge a town over. Average high schooler? Make them have a dispute with their parents, or have a movie marathon. Witch? Have them practice a few spells when they're alone. Maybe confide to their friend about their magic. Werewolf? If they've already triggered their curse from the start, have them struggle with turning on the full moon. If not, have them trigger it in an unexpected way.
Yes, your OC will have to be included in original scenes. When that happens, give them their own lines. Stealing a few here and there from a canon character is fine. Just don't overdue it. And if your OC needs to replace another character, rework the dialogue and the scene itself.
What I'm trying to say is, please make the TVD world your own. But also the same.
It's confusing. It's hard. It's also really fun.
Good luck, and happy writing!
-Jordan