Board Thread:Lore Discussion/@comment-11345660-20130908095939/@comment-3573184-20130913105559

Vyrthur does say however

Arch-Curate Vyrthur "How dare you. I was the Arch-Curate of Auri-El, girl. I had the ears of a god!" Serana: "Until the "Betrayed" corrupted you. Yes, yes. We've heard this sad story" Arch-Curate Vyrthur: "Gelebor and his kind are easily manipulated fools. Look into my eyes, Serana. You tell me what I am."

Which implied he made his bother belive it was the Betrayed who turn him.

And while he could have been turned before, the presence of the vampire Falmer, and the emphasis on the attack throughout the Forgotten Vale part of the Dawnguard MQ, makes it seem more likely to me it was around that time.