Until I was researching a different problem I was unaware that we should be able to hire the Fampyr following completion of The Champion of Berath. I tried during my second playthrough but was unable to get the option to hire her; then all of my save game files disappeared so I had to start over.
After completely reinstalling the game I started over for the third time. After I received The Champion of Berath quest, I went to Raedric's Hold, ensured I had an open position in Stronghold - Hirelings and saved. Then I had the Fampyr take me directly to Lord Raedric and defeated him and his minions. After going back down the stairs to the Fampyr I did not get the option to hire the Fampyr. All I got is the response, "You have your victory, champion." Tried leaving the area and coming back, saving/reloading but still only got that response from her.
After a little research, I saw some comments to the effect that to get the option to hire the Fampyr, the "A Servant of Death" quest must be either Active, In Progress, or Completed. So off I went to Elm's Reach, started the "A Servant of Death" quest and returned. Once again I had the Fampyr take me directly to Lord Raedric where I killed him and his minions. Once again, the only response from the Fampyr is "You have your victory, champion."; but no option to hire her.
Next I reloaded the save and traveled to Elm Shores to find the High Ovate Erona. After talking her into killing herself I returned to Raedric's Hold, saved, and talked to the Fampyr again. Once again I had the Fampyr take me directly to Lord Raedric where I killed him and his minions. Once again, the only response from the Fampyr is "You have your victory, champion."; and yet again, no option to hire her. Tried leaving the area and coming back, but still no option to hire the Fampyr.
Finally, I reloaded the save I made after getting rid of the High Ovate, returned to Elm's Reach and killed the Archdruid Rehstin. Then I went and talked to Berath and completed the quest. Returning to Raedric's Hold, I once again saved and had the Fampyr take me directly to Lord Raedric where I killed him and his minions. Finally, as a now acknowledged Champion of Death, the Fampyr asked to join my forces. So, to make a long research project short, "A Servant of Death" must be COMPLETED to get the option for the Fampyr to join your forces!