From the wiki:
If you convince the vampire hunters to leave, they actually remain in Skingrad forever, although the Journal states that they have left Skingrad.
The above is likely what happened. You actually advanced the quest and could talk with the Count, but didn't realize it. What does your last Journal entry say.
Also from the wiki:
If you kill the vampires on your own after talking to Eridor about them, you are unable to subsequently convince him to leave without murdering him. To avoid this, wait until you notice the map markers of the hunters in Bloodcrust, and then proceed to either help them, murder them, or let them be killed by the vampires.
* PC Only This bug is fixed by the Unofficial Oblivion Patch.
Something to watch out for.
You need vampire dust from a bloodcrust vamipre, not just any vampire when you tell him where they are. They normally leaves for the cave about 1am....
I thin this is false. You do need the vampire dust to convince them that you've already killed the vampires if you take that course of action. However to convince them to go to kill them I don't think you need the dust.