Nice start of a story.
But since Harry knows of the prophecy, he has the upper hand over the Vampires until Voldemort dies.
a) Take the hard path and figure everything out by himself. That means the horcruxes, training, getting used to being a Vampire, finding out why he was turned, ...
b) Blackmail the Vampires into doing everything for him.
- Take the portkey
- Tell them the prophecy. The Vampires know they are screwed if he refuses to kill Voldemort since they are smart enough te realise that Voldemort will kill them when they outlive their usefulness.
- Force them to bring the person that turned him and his 'employers', then decide what to do about them.
- Force the Vampires to find the Horcruxes and train him for the final battle against Voldemort
If I had to chose between a and b then it would be an easy call. Hard work or doing nearly nothing...
Since there is word about employers etc, I'm going to assume that there is another prophecy in play...
Author's response
An interesting idea, but there are a few problems with option B.
1. Harry doesn't know if it's really a Portkey, or what's waiting on the other side. He doesn't know Darknight's motives, and the portkey could drop him into a spiked pit for all he knows. Or the vampires on the other end may not be as receptive as Darknight implies. Not even Harry's that stupid.
2. He doesn't know what kind of capabilities the vampires have. Yes, he knows that they are immune to the AKs, but he doesn't know just how powerful they may or may not be. And if they have sufficient resources, they may not even care if Voldemort turns on them, since they could repel any attack that he makes.
3. That's assuming that the vampire Powers That Be know about and sanctioned such an action. Darknight could be part of a splinter faction, operating outside the vampire public.
4. See point 2.
So all in all, a decent idea, but at this point in time, makes far too many assumptions to be safe or effective.