Vampires. Stalkers of the night, feared by commoners but romanticed by poets. Monstrous bloodsuckers, beautiful seducers or elegant lords. What do they live from, where does their condition come from and what are their motives?
What does this condition do to you? When people have urge to eat something, they have a shortage of some nutrient. Vampire dicease is killing hemoglobine what makes your body to want fresh blood to cover the loss. Animal's blood will do ok, but best source to replace bloodcells vampire dicease is eating away is to get human blood.
These vampires don't have pointy fangs, but they use other means to get blood using sharp tools for it. They may get a trusted person(s) to donate them blood, or they might have to seduce their victim. Unlucky one's must take it by force.
This dicease is a hard one in middle-ages. You cannot tell about it, as it might be viewed as curse and blashemy. If you told it, you would be quaranteed in fright of contagious dicease.
You are a loner with your illness and you must hunt your pray. Even if it is taking the blood against another's will or not. After all, it would be suspicious if you did buy big amounts of animal blood from butcher.
Also see: http://emedicine.medscape.com/article/957765-overview
Best way to prevent evil person to come back from the grave every night to pray on human blood is to hit a stake through his heart, chop his head off and stuck his mouth with garlic. But if you didn't prepare the body right and it is now a vampire, you better find it's lair before it is too late.
At some point hunger has grown so great, you cannot resist to take a sip or two, but start to drink people dry. Then rumours start to roam. To survive, you must be intelligent, witty, cunning. If you live long enough, you start to build your own reign of terror. You are a rumour for outsiders of your village, but for villagers, you are a nightmare. Maybe people start to be too scared against your power you have built to try to kill you. Maybe people even offer you sacrifices of travelling men to offer blood to satisfy your hunger so you will leave them alone for another night. But your power has a cost, your reputation stretches wide.
Those who aren't intelligent, witty and cunning enough are doomed to be slain again. This time for good.
Class in society
For a vampire, blood or else where they get their power and nutrition from is all what matters. Everything else comes second. They are predators with eternal hunger. They might play with you, build social network and enjoy balls, or even build their castle but behind that all is a monster. It might be conscious choise or subconscious instinct to use these matters to cover their true nature.
Vampire can be a beggar in the street, as in poors district it is easy to hunt. No one in authority or status will miss your victims. Or you might take your higher class life back, or use your cunning powers to rise high. It doesn't matter do you live in sewers or castle, you hunt and people you associate with are your pray.
Vampire might enjoy his solitary life, or being a center of attention.
How to give flesh for vampires?
In fantasy games vampires are usually monsters player characters slay like any monster in Monster Manual (or similar). Make vampires more than just stats and that evil dude at place X you need to slay for a quest. Make vampires scary predators in the streets, or horrible beasts at countryside. Or a count, cunning and influentic.
Use your imagination. Vampires might be a bit boring, but make them interesting.
I'd like to have your ideas of different kind of vampires in comments!