What nobody seems to have mentioned here yet is that there are rules for a Swarm of Vermin, published in the Chronicles of Darkness main book.

The Trained Guard Dog profile (p.121) "Doubles as: K-9 hound, wild animal, swarm of vermin, genetic experiment."

So the Vampire's attack dice in Swarm Form should be 6, in the same way that if they become a cat their attack pool drops to 3 or if they become a Badger it becomes 5.

6 dice to attack, ignoring defence, while gaining the massive defensive benefits of Swarm Form still makes it a bargain - especially...