The feat "Become the Swarm" of a vampire. HD and size

The question is quite simple, but I have looked and can not find a clear answer.

Let's say that a 10HD vampire has the following feat, does he calculate the damage and the size through the Table: Swarm damage by size in the swarm subtype link that follows or not? and now that I think about it, do you also add some skill bonus to the swarm form damage?

Feat:
https://www.d20pfsrd.com/feats/3rd-party-feats/fire-mountain-games/become-the-swarm/
Swarm Subtype:
https://www.d20pfsrd.com/bestiary/rules-for-monsters/creature-types/#TOC-Swarm-Subtype

Any help would be appreciated, thanks.