I don’t have a vampire oc but, sharks, may I pitch alma ferrera:
human. mom was a sorcerer who had a partner, bellamy, who took alma as her apprentice (how her mom and bellamy met).
her mom died when she was a tween. she attempted to bring her back to life with high level, forbidden dark magic. bellamy knows alma was researching. secretly allows alma to try it and also inconspicuously supplies her with the necessary materials. alma thinks it’s a resurrection spell. bellamy knows it’s a exchanging your life for theirs spell. when bellamy “stumbles” in on alma attempting the spell and sees it working, she intervenes, guilt ridden, at the last moment as she watches alma’s flesh and bones deteriorate and ash away. alma’s physical body has been necrotically taken. only her soul remains. before her soul is obliterated, bellamy binds it to a suit of armor (in modern verses, she makes a risky hermit-crab-like exchange into motorcyclist garb (leather jacket and pants and helmet and gloves and knee pads and shoes)).
alma is functionally immortal. so long as she stays bound to the armor/motorcyclist garb. she wanders “alive” on the mortal plane away from her mother in the heavenly afterlife because she fears attempting forbidden “sinful” magic would destine her for hell, away from her mother forever.
she is hundreds and hundreds of yrs old.
her only intact senses are sight (she can see 360 degrees through the slits of armor) and hearing. she is capable of speaking. she has unlimited endurance because she does not possess a physical form to exhaust. all limits of hers are mental (e.g. she could stand in place for days without exhaustion but it’d be incredibly boring so in that respect she might tap out).
she doesn’t require sleep and is incapable of sleeping. for a modern readaption, I’ll say that instead of elves teaching her how to trance, vampires did. it’s an usual and complicated skill for a soul that’s technically still human and mortal to learn. but within a yr, she masters it.
this is the only way she can restore her natural mana (previously as a regular human she had to sleep).
she wanders the world in search of bellamy. for revenge?
a location spell incorrectly leads her to bellamy’s daughter (dropped off at a magical boarding school; bellamy rarely visits)—the exact age alma was when she attempted the “resurrection” spell.
alma takes the daughter as hostage. this organically turns into an apprenticeship.
the daughter becomes of graduation age.
alma accompanies her to a magical assessment—something all magic users must pass and be a card carrier of to legally do magic.
they don’t acknowledge alma as a valid teacher because alma never got her magic license.
so they take the assessment together. daughter almost dies. alma does the “my life for yours” spell exchange to save her, thinking she’ll die doing so but doing it anyway. alma doesn’t die because she’s lived hundreds yrs longer. it only drains her soul of the equivalent amount of yrs the daughter has lived. they both pass the magic assessment.
daughter becomes the age that alma’s mother was when she died. they learn bellamy only had her as a sacrifice to use “my life for yours” equal exchange for alma’s mother to live again. bellamy has been physically and emotionally distant from her daughter so she has the fortitude to follow through on the exchange spell.
alma and the daughter flee. etc etc etc