Blood is Thicker

A billdip Vampire Au.
This wasn't the first time a vampire had breached the first clause of the treaty, and it certainly wouldn't be the last. It was a wonder that the peace still held in spite of the fact, but then, it was one that neither side wanted to see abolished. It held only because of the many other clauses and agreements between the two peoples that not even the most lawless of the blood drinkers would dare disrupt. Dipper never liked it, but the treaty was hard won, and once it was broken, it would be near irreparable. But now they'd stolen the wrong person, and Dipper would make sure they knew it and committed his face to memory. Not even the treaty had been able to save his sister from the threat posed by lone rogues.
The lines between the monsters he'd pegged all vampires for and what he's learned about them from the temporary alliance begin to blurr, however, after he joins forces with the head vampire, Bill Cipher.