Hmm... Level 34. When I last fought Dorian, I was at 31 and I grabbed a lot of things to prepare myself. I am not sure how hard this "finalized" Dorian is, but I am hopeful that I can still beat him before chapter 5.
On that note, I thought the fight would still be "unbeatable" if you challenge Dorian before chapter 5. Like, it is designed that you can't continue past this point unless the player reaches chapter 5. But if you do reach chapter 5, you're locked out from the vampire route. I think this because the further you go into the main story, the vampire route makes less and less sense. The fates of certain characters wouldn't sync correctly and certain plot points surrounding the MacGuffins would become plot holes.