I was sent to count of skingrad by the mages guild. He requires me to get rid of the cave of vampires and the vampire hunters before he will give me info for the guild. I killed all the vampires in the cave ...and tried to talk to the main vampire hunter. but it wont let me tell him the vampires are dead. How can i get them to leave without killing them?

When you talk to Eridor (head vamp hunter) you want to have a portion of the special vampire dust from Bloodcrust Cave in your possesion. You can tell it is the correct dust because it weighs 0.1 intead of the normal vampire dust 0.2. With that in your possesion, he should agree to leave. More info here: http://www.uesp.net/wiki/Oblivion:Information_at_a_Price

make sure you grab the dust.

see here for more details. http://www.uesp.net/wiki/Oblivion:Information_at_a_Price#Killing_the_Vampires_and_Getting_the_Hunters_to_leave

Edit: ninja'd

thank you very much for your replys. I am still running in circles and very frustrated. I went back to the cave 3 times to make sure i didnt miss anything.. I told him originally where the vampires were but he didnt go there so i went and killed them. I got a message that now i have to deal with the vampire hunters. i went and talked to him...with 10 units of vampire dust all .01. and all i get is the same questions i can ask ...that i got in the beginning... i am so frustrated. i know i am missing something but what?

It has been a long while since I have completed this quest, but I seem to remember having a similar problem to what you are describing. I think I reloaded an earlier save and followed the walk through in the link Acadian and Pupp provided.

Praise Alkosh!

In that quest I talked to Eridor about the Bloodcrust cave and waited near it. After a while (it takes a few hours) I saw them arrive and go inside. I let pass a few more minutes before getting in. I got the message that they were all dead. After all they are vampire hunters... Before I tried to fight with them but no way.. They had to die.

I'm grasping at straws here: I assume that while you were talking to Eridor after clearing Bloodcrust Cave that you checked to see if the dialogue options you need require you to scroll down? I don't recall if scrolling down is required to find the right dialogue option in this quest but it can be a frustratingly silly quest stopper in some others (It got me the first time I did Unfriendly Competition).