Nightingale and Frank Caffrey dealt with a vampire nest in Neasden. Curiously these turned out to be infected rats …
That's such a good idea it deserves to be stolen. There's even a small precedent:
"I heard a commotion one day in the yard,” he said, “and there was a young turkey thrashing about the yard with two rats hanging to its neck. I ran into the yard and drove them away and found that they had sunk their teeth into the back of the turkey’s neck and had been sucking the blood. The turkey was strong, but the wounds poisoned it, and swelled its head and I had to kill it. The worst of it is, these rats won’t be poisoned, for they refuse to eat raw meat or cheese that has been fixed for them."
Also, a very fun movie, Mulberry Street. I had a DVD copy but it rotted. I shall have to get another one somehow.
In Night's Black Agents rats get group stats, which increase in stacks of 10. For every 10 rats added to the horde, its Health, Athletics and Damage all go up. Otherwise its ability pools and damage are fairly unremarkable. It works best in large numbers, which is why Steven King wanted to use them in Salem's Lot, but was persuaded against it because it was felt the scene was too gruesome. "I had them swarming all over him like a writhing, furry carpet," says King, "Biting and chewing, and when he tries to scream a warning to his companions upstairs, one of them scurries into his open mouth and squirms as it gnaws out his tongue." King was drawing on a similar moment in Stoker's Dracula, where a horde of flesh-chewing rats are neatly disposed of by Godalming's dogs.
To make this work in-game, the Director needs to answer an important question. Did this happen by accident or design?
If accident, then there must be something that caused this. Perhaps the strange pathogen that created vampires first came from rats, and some of the breed still carry the strain. Or the chemicals got out of the vat, the rats ate a vampire in its coffin and gained some of its powers, what have you. The key thing here is that to understand, and beat, the pack, the agents will need to discover what the accident was, which itself is at least one or two scenes' worth of investigative work. If they don't do this, then the likelihood is they won't exterminate the threat. No matter how much damage they do, one or two of the infected will escape, to carry the infection to another pack.
Once the agents understand the accident, they can devise some kind of cure, perhaps by introducing an antigen of some kind to the wider population. Then the rats solve the problem themselves, by passing the McGuffin on to the rest of the pack. Or perhaps they just spray holy water on anything that moves and most things that don't.
If it was by design, then there has to have been a purpose. Perhaps the vampires intended to use the rats as emergency blood packs, spies, assassins, saboteurs. A magical strain of vampires might give these rat packs to favored human servants as animal familiars; after all, the whole point behind the fabled witch's mark is that the familiar sucked blood from its master at that point. Whatever that purpose may be, the agents need to understand it so they can beat the rats, or work up some kind of defense. If you know that rat saboteurs might come after the cables and wiring in your safehouse, put down lots of rat poison laced with vampire banes. If you know that these rats are magical familiars, then you also know that blooding the witch will cause them to run away in terror.
Next question: how powerful are these meant to be? Answer being, not very. These are minor antagonists at best, not boss monsters. I'd recommend treating them like inferior Renfields, so where Renfields get 12 points to add to abilities, vampire rats get only 4, no bonus to alertness, +1 melee damage, no free powers, one other power, aberrance starts at 3 and goes up by 1 for every 10 extra rats in the pack. Feats of Strength probably aren't an option, but otherwise pick the one you like the look of. Vulnerable to the same banes and blocks as the vampire that created them, and if that means they explode in daylight, well, now you have kamikaze exploding vampire rats to worry about.