To become a powerful, sun fearing vampire you can do two things. One is to go and talk to your good old Dark Brotherhood Friend for that little present he was going to give you. Unfortunately if you killed him already in the Purification quest, you just have to find the vampire, so they can hurt and harm you untill you are infected. You can find 5 or so vampires by doing a certain Daedric God's Quest... Another bad thing is that I forgot which god.... =( so just walk around looking for Shrines, and you will hopefully stumble across this quest.