The Special Requirement to Hemomancy
Vladimir mentions that he is the "universal recipient". I realize that nearly every type of magic needs some apprenticeship. Ryze studied under Lilith, and Syndra formerly had a master. As for Vladimir, there was a special requirement for Hemomancy: the AB blood type. A person with AB blood is capable of receiving blood from the other blood types. Vlad would have failed his master Dmitri's final test if he was unable to absorb all the blood from him. Given that the AB blood type is slightly rare in IRL, I presume that Hemomancers are rare in Runeterra because potential Hemomancers need sadistic streaks and AB blood.