Vernossiel was the commander of a small Scoia'tael faction in Velen that was attacking Redanian army supply transports during the Third Northern War, reasoning that it wasn't civilians they were killing but soldiers who killed, tortured, and raped their fellow brethren.
If Geralt kills her: she met her end in Velen, when Geralt killed her and her small band.
If Geralt lets her be but then tells Felix Grubb about them: presumably her group was attacked by Redanian soldiers though she managed to escape. Knowing Geralt was the only one to know about them, she later tried to ambush the witcher near Novigrad, resulting in her death.
If Geralt lets her group be: she continued to attack supply caravans with her group.
- If Vernossiel and her unit are killed at their camp, leaving and then returning to the camp some time later causes all of them to respawn.
- If you decide to betray her she'll try to kill you near Glory Gate.