Pathfinder – What HD and size does the "Swarm" feat of a vampire give?

The feat of Swarm is made explicitly, that the points of blow of the swarm and those of the vampire are identical:

This swarm has a number of hit points equal to the vampire, and any damage done to the swarm affects the vampire's total hit points.

Except for the hit points, the statistics block becomes the swarm for combat:

A vampire can not use any of his natural or special attacks. […] the movement, the natural weapons and the extraordinary special abilities of the swarm in which it has been transformed wins.

[It] It retains all its usual special qualities.

So a 10-HD Vampire becomes a swarm of 10-HD bats with all the natural abilities of a 10-HD swarm, with the exact abilities specified.