I'm having the same issue... I don't have the option of choosing the vahnati to blame when speaking to either Erika or Ostoth. I have completed the Troglos/Giants line of quests as well as the Golems line of quests. Both Anaxamander and Erika say to me that it's the Vahnati, but I can't say that to Erika or Ostoth. Am I missing something?