I was curious about this for a while, and searching for this question led me to no real answers. So, here it is, and my test results (which surprised me, to be honest):
What happens to the Radovid Murder Plot, if Ves died during Eye for an Eye?
I had suspected that this would lock a whole lot of things: Roche/Ves not going to Kaer Morhen was the obvious thing; but I also suspected that it might have an impact on A Deadly Plot (because Roche might be sulking) and on Reason of State - basically, dooming the entire north to the reign of the Mad King - millions of people suffer because one stupid girl did a suicide mission and wasn't rescued.
This hypothesis was, luckily, not correct.
Here are the longterm results of a dead Ves during Eye for an Eye:
- as expected, Roche will not go to Kaer Morhen.
- A Deadly Plot happened just like it always does, no sulking, no mention of my failure.
- Reason of State happened, but things were a bit awkward.
That is:
- in the first cutscene, Roche normally says "and Ves with a well-oiled crossbow". This was changed to "and
someone with a well-oiled crossbow".
- once on the bridge to Temple Isle, I noticed a Ves NPC fighting with her sword, like she normally does.
- once in Madame Irina's place, Roche, Geralt - and Ves - approached the waiting Thaler.
- then they toasted "to Temeria": Thaler, Roche, Geralt... but not Ves. Ves was silent.
- Ves remained silent but present for the rest of the cutscenes and the fight.
- her biography entry in my log still says that Ves died during Eye for an Eye. So, I suppose, this wasn't Ves, but her twin sister, who also happens to be called Ves. Or maybe it was a doppler.
(I went for "Nilfgard wins" because I normally always go for "Dijkstra wins", and I wanted some variety for this horrible vanilla state which I want to move into B&W so see Dandelion after a completed vanilla instead of in a pure B&W game.)