In order to create a replacement “Master Vampire” after his is accidentally killed, a perfectionist thrall has 48 hours to steal and consume the blood of 100 virgins from a mobile high-school blood bank, before he and his master’s other sons’ powers wain, and they succumb to mortality.

Yes must steal enough blood to protect his family? sounds better and yes "caretaker" or "loyal caretaker" also sounds better.

But I still feel like just steeling blood whether to feed his family or himself is not compelling enough of an action to sustain my interest in this instance. The fantasy-Goth aspect of vampires aside the story is about a guy struggling to provide for his family like Will Smith in The Pursuit of Happiness. Will Smith's character is a struggling all round good guy underdog, however your MC is a natural killer, all be it rejecting his nature but a natural killer none the less and an outlaw so in a way he's an anti-hero.

I think that an anti-hero struggling to provide for his family is less interesting than an anti-hero fighting a bona fide bad guy. Mostly because I can't see the tangible goal to end the problem he has, that is "to provide for his family" or the equivalent in vampire terms "... steal enough blood to prevent his family's death."
How long have they been starving for? How long can they go without blood? How many of them are there? If he gets the blood he needs now how will he feed them in a months time?

I don't think these questions should be answered in the logline but are inherently present after reading the current draft of it.

I would suggest shifting the focus of the logline away from the specific form in which he provides for his family and onto an easily understood imminent threat. Perhaps in his pacifism he befriended a person that works for the blood bank who sneaks out bags of blood for him and his brother/family.

Then after his master is killed their lives and source of food are threatened and he must fight to save them all.

I like fantasy and horror films and think this is a nice and refreshing take on Vampire stories (un like the horrid treatment the genre got in Daybreakers and other poorly structured films) so definitely worth perusing.

Thanks so much for your help on this so far man.

I hear what you're saying re: linking the antagonist to the goal / stakes, but I think it sounds clunky when I attempt it. I don't think "to prevent his family from being hunted as well" rolls off the tongue. Perhaps it just needs to be changed to "must steal enough blood to protect his family" ... more needed you reckon?

Also - I can find another word that is less accurate to describe the character ... but he is 100% a thrall, a servant, as per pretty standard vampiric lore. That's what he is, and I'm OK with calling him servant for the sake of accurate communication, but I think anything else lies about the nature of his function in his world. Perhaps 'caretaker' could work?

This reads much better as it's leaner, the stakes are clearer and a pacifist vampire sounds an interesting combination. It's similar in that sense to Brad Pitt's character from Interview with the Vampire.

Whilst a pacifist vampire that refuse to kill and needs to rob a blood bank sounds interesting I still think that the stakes need to be directly related to the antagonist somehow. By doing this you put the act of stealing blood in a unity of action with that of defeating the bad guy and make it a far more compelling story.

"After his master is left for dead by a brutal hunter, a pacifist vampire's servant must steal enough blood to prevent his family from being hunted as well."

I get that servants a probably conventions of the vampire world but I still can't shake the negative connotation to the word servant/thrall or maybe that's just me. Is there a better position you can give him?
Loyal butler, personal physician, immortal companion, eternal tennis partner...