I don't know how far along you are, but it occurred to me, in order for this to be neither overpowered or annoying, the two states of fed or unfed and the transition has to be not annoying at both extremes.
Just a thought then, that after killing/feeding, the vampire has more normal human likeness and traits and stats (fat and lazy), but as they get hungry you have a tradeoff of one thing versus another, for example, social interactions become affected (trading, discourse, being in town), health gets lower (vulnerable) but stats go up and spells become available, so it becomes, from a design point of view, risk vs reward slider.
That concept would make it interesting to play as long as either end wasn't extreme, so perhaps in order to get a discount on your deals you kill someone first to get a "buff", and before a fight, go hungry for a bit for some burst.
It's a heck of a lot of status effects to manage, i'm playing with little ones right now, but just a thought. Keep going with it mate.