I had vampirism right before I had Dawnguard and all I had to do was go to a shrine and cure myself. If that doesn't work, and you need a daedra heart, go to Dawnstar. There'll be a guy who set up a museum talking about the Mythic Dawn. He'll ask you to go to various dungeons to find the pieces of Mehurune's Razor. Be warned: The fights are for a little higher levels. When you finally collect all 3 pieces, you need to go to Mehurune's shrine. Afterwards, you can fight the dremora the Mehrunes sends on you. I know it's a roundabout way, but that's how I got the daedra hearts. I also went full beast on them and mauled them, too, so there's that.