So reading through the comments, this is how I did it and had no issues.
Saved Siva from the gallows, went to her house and she took me to her basement. Talked to her, opened her drawer of ingredients, did the ritual.
Then I went and got my second source point (mine was from Mordus). Went to Cloisterwood and got more blackroot, went to Siva, did the ritual again. Got Source Vampirism, no voidlings yet.
Got my third point, went back to Siva, did the ritual again. This time had to fight 2 voidlings in the Hall (one strong vs magic, one immune to physical). After the 3rd ritual, popped back into Siva's basement from the Hall and you get a script about "something smelling wrong" or "something isn't right". Then those same 2 voidlings from the Hall show up and murder Siva, and fight you again.
After that, use spirit vision to talk to Siva, she tells you what to do next and to go tell Malady (I haven't done this yet as I'm still finishing the rest of the quests).
Now for people who can't find Malady, I don't know if you remember but she told you to talk to the ship when you are ready to summon her. "The ship knows how to get in touch with me" or something. So go back and talk to the front of the ship again (like you did to wake it up and leave Fort Joy). I assume at this point you will get an option to summon Malady back to the ship.