This system isn't utilised because of lack of clarity and feedback making it difficult to RP and as a result undesireable. My suggestion is as follows.Vampires can recruit other characters as their thralls by feeding them a portion of their immortal blood. This confers physical improvements to the thrall, as such:
First feeding in 24 hours: +1 con
Second feeding in 24 hours: +1 con
Third feeding in 24 hours: +1 con
Fourth feeding in 24 hours: +1 con
These bonuses are cumulative and count towards the +12 ability cap.
The more a thrall feeds from the vampire, the more urgent the thrall's dependency on the vampire's blood becomes. A prolonged period without feeding may result in drastic withdrawal systems, ultimately culminating in death if the fortitude saves are failed. (Reports of this system being bugged have been made, resulting in there not always being a death save from vampire blood withdrawal.)
A Non-Vampire drinking the blood of a Vampire grants them 100% Blood. This grants them +4 Soft Con.
Their Blood % reduces with heartbeats, each 20% reduction they loose 1 Soft Con. At 0% they gain a reduction like the Death Penalty. They can either let the penalty timer rune down and attempt a save vs death. At which point they are "Free" of their addicition to the Vampire, or they can feed again and restore their blood to 100%.
The catch is, each time they feed, the rate in which their blood % reduces increases. Meaning the more they feed, the quicker they'll hit 0% and eventually roll vs death.