If I were going to have it turn victims into minor vampires, I'd certainly raise the cost.
I'd probably also want it to be a skill that it gets in place of attach when it has a host.
"Turn: If host has less HP than Vampiric Parasite (or maybe if host has less than 3 HP), host turns into a Minor Vampire and Vampiric Parasite detaches."
That way you could just slaughter your opponent's creatures and slowly turn yours into vampires to regain health.