I was going through the quest Open Sesame! when I ran into Countess Mignole.
She asked about Vesimir and I responded that he died a hero.
What happens if you initiate this conversation before he dies? Do you only get new dialogue or do you get a quest to reunite the star-crossed lovers before he dies? Unfortunately the linked wiki doesn't discuss this even though it does discuss different options in a later quest Whatsoever a Man Soweth.... This makes me think that the devs at least explored the idea of you getting to Open Sesame before meeting up with Ciri, and might have opened up the possibility of reuniting him with Countess Mignole.