Talk to Isla, then head out toward the fort on the area with the cult and temple, at the fort buy a cod piece, fight and beat the vampire on the world map, get thru the whole conversation. Then return to Isla and converse with her. They'll get into a 3way and you'll be unable to join but you'll leave it with mixed feelings. My thoughts mostly were that Isla have basically given up on killing demons she's attracted to.