The Vampire and Progenitor are the Baby's unique skills.
Most of the reincarnated students got their unique skill from a certain aspect of their past lives.
- Kumoko
Kumoko's character in a MMO was only oriented in speed and attack. Hence why her unique skill, Skanda gives her speed.
- Baby
In her past love she had a heart disease that made her feel extremely exhausted on the mornings and combined with her bad cares tooth made the other students call her Rihoko (Real Horror Kid).
D went further with her fuckery and made her a true progenitor. In the first place vampires in that world are extincted because the other 3 races (Human, Demon and Elf) hunted them down. Vampirism is treated as a plague mostly because most vampires just go berserk after their transformation.
Also Vampirism being a disease isn't tied to a race, that why the babies race is Human Vampire. And to be a progenitor you don't need to be pure blood just being born from a vampire.