If you question yourself, you already know the answer.

I agree with the Stockholm suggestion, it's a very effective way of telling the story. By establishing the relationship of the two characters early in your head, it will be easier for you to write the resulting conflicts. Otherwise, you won't have natural plot ideas, but because it you need to put something in the story.
If you take that tactic, you will have to be prepared to make in more of a psychological story. It means there might be less obvious magic and more manipulation.

As for crazy Harry suggestions, I don't know. I'm sort of writing that now, but it's not going to useful for you. And I just read kmfrank's and it was awesome, but like it was pointed out, readers can be stupid. The best suggestion I can give is to look at other examples in movies and tv. They always seem to have a few specific characteristics that define them as insane. For instance, River said odd things at times, but she had a physical way of holding herself that portrayed her mental damage as well.

And remember, crazy people have logic, it just that most people can't understand it.

 

That is all.

 

To me this seems like the makings to a crack fic.

When it comes to vampires the best generally are all about power, control and most important as of recent decades dark sexuality.

Now a vampire making Harry into their bitch, well it is overdone and usually something found in slash fics, sure the fan girls and yaoi lovers would like it so really it depends on who you are writing it for and if you can and do write that type of story.

If I were to write such a fic, Harry's unfocused magic would attract the vampresses attention at a distance then within a certain range a weak one way parasitic connection would form, further drawing her to him. Something occurs that causes Harry to leave the Dursley's and she capitalizes on the opportunity that is created to feed from him creating a blood bond of some sort.

At which point to further bond them intercourse would be needed and thus cement them together.

As far as the vampress is concerned yes she would have to be magical for such bonding and different from the nonmagical undead, as they could be just due to a virus or demonically possessed corpse than magical creature.

Now the bonds between them could influence both of the bonded in a number of ways. For Harry his magic and emotions could start to shift to be an animalistic dark similar to an vampires. This would create a gradual shift and not just suddenly turn him into a dark lord or terrorist. For a more subtle and insidious cumulative effect. Also Harry might not be such an good guy to begin with, he simply wears the 'golden boy' mask at school and to avoid trouble with authority. This could be used to explain a more dramatic shift in his personality once he is bonded.

For vampires to exist regardless of the ministry, the nonmagicals would have to have a high turn rate and be somewhat troublesome for them to be heavily hunted by anyone other than dedicated hunters and organizations, thus the ministry is not likely to target them too often. As for the magicals they would have to be too tough and troublesome to prevent being wiped out. Such would imply the existence of dens, covens and possibly a hierarchy. For that a good number of such dens would be needed, maybe each would only have up to a dozen or so vampires in them, rarely more because that could lead to making them a good target for hunters.

The Magical vampires dens would be hard to find let alone take down without great force and resources. When out alone or in groups smaller than four then they could occasionally be targeted by the ministry and or groups of wizards, yet it would take maybe 20 or more skilled wizards to successfully eliminate them without taking too many losses or risking too much. Nonmagicals would be much easier as a single wizard could target an entire group of them with a fire spell and destroy them without much risk, thus nonmagical vampires would not intentionally target magical beings. Another part of the reason for them to avoid magical beings could be that the magic within them could act out in defense even unintentionally and destroy the vampires.

Though for anyone writing a crack fic or appealing to the slash fans, it really matters little, doubt anyone would notice something like plot and technicalities underneath the smut.

 

Firstly, it is, for all intents and purposes, a crack fic but god damn it, its not a smut fic! Smut does nothing for the plot and if it weren't for the popular culture vamp, I wouldn't even consider it! No smut!

Secondly, I don't think there is anything in canon that says vamps are being exterminated. They are second class citizens just like the Giants and werewolves. I thought about making an underground community but that would be more of a vamp story with Harry Potter, rather than a Harry Potter story with a vamp in it. Which I do not want.

A blood bond I will use. As this is before the DoM fiasco, I assume Harry can go for a walk and be jumped without readers feeling jipped?

 

Even better, how about he's somewhere he isn't supposed to be and then he gets jumped?

 

The devil is in the details...