What HD and Size does a Vampire’s “Become the Swarm” Feat give?

The question is quite simple but I’ve looked and can find no clear answer.

Lets say a 10HD vampire has the Become the Swarm feat; do you calculate the damage and size via the Swarm Damage by Size table in the swarm subtype, or do you not? And now that I think about it, do you also add any ability bonus to the swarm form’s damage?