Can I solve "Berengar's blade" without talking to Vesimir just before the battle at Kaer Moren?

I am at Kaer Moren after finding Ciri before the battle.

Inside Kaer Moren, I found a chest whose loot started "Berengar's blade". After reading Berengar's notes, it tells me to talk to Vesemir.

Yet talking to him does not work as it's only pre-battle talks. (Geralt only tells Vesemir that he seems to be in a good mood, no real interaction happening.)

When I progress to start the actual battle, the quest fails immediately. So while this quest seems to be glitched, I wonder:

Is there a way to solve this quest without talking to Vesemir so to only have this part of the quest fail and not its entirely?

Solution 1:

Yes, you can just go to the cave that contains the diagram without speaking to Vesemir. The catch is that you don't know where it is!

Location:

Follow the river downstream until it starts to head north and there is a cave containing the diagram (and a large chort!).