It's should be freeze of aging like vampires? unless they want make vampire only kind of freeze of age and immortal while Potion of Immortality gave you not die by aging but still age (it's would be decay, not true immortality) for gameplay balance, they might not want vampire become useless.
I hopeful it's replace of Potion of Youth, but I guess not, it's make all Potion of Youth, Cow Plant and Age-Away serum useless while same time you had to keep drinking those, if you want stay young forever, making Potion of Immortality useless unless you want elder forever, that's make vampire seem a tempt to become vampire as vampire had both by stay young and live forever at cost of eating/drinking plasma.
I guess it's all gameplay balance?